Controller for controlling printing on both surfaces of a sheet of print media

ABSTRACT

A controller for controlling printing on both surfaces of a sheet of print media includes a first print controller for controlling printing by a page width printhead of a first print engine and a second print controller for controlling printing by a page with printhead of a second print engine substantially simultaneously with the printing by the printhead of the first print engine. A first communications link interconnects the first print controller and the second print controller for synchronizing the print controllers. An second communications link is connected to at least one of the print controllers for interfacing with a host system for receipt from the host system of descriptions of pages to be printed on said surfaces of the sheet of print media by the print engines.

FIELD OF THE INVENTION

This invention relates to a printing system. More particularly, theinvention relates to a controller for controlling printing on bothsurfaces of a sheet of print media and to a method of controllingprinting on both surfaces of a sheet of print media.

BACKGROUND TO THE INVENTION

Most prior art printers comprise a print controller for receiving printinformation from a computer. The print information is usually suppliedin a page description language, such as Adobe PostScript. The printcontroller takes the print information and performs operations on it inorder to convert t to a form suitable for supply to one or moreprintheads. These operations can include, for example, decompression ofcompressed data, rasterization, compositing and scaling. Most printershave a print controller that supplies processed data to a singeprinthead.

In duplex printers, both sides of a page need to be printed. Often, thisis achieved by printing one side of the page first, followed by theother side, sometimes using the same printhead. In other embodiment, apair of printheads are used, one for printing either side of the page.In U.S. Pat. No. 5,784,077 (Silverbrook), a pair of printheads aredisposed at different points along a paper pathway.

Prior art arrangement make registration between information printed onrespective front and rear sides of a page relatively difficult toachieve.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided acontroller for controlling printing on both surfaces of a sheet of printmedia, the controller including:

a first print controller for controlling printing by a page widthprinthead of a first print engine;

a second print controller for controlling printing by a page widthprinthead of a second print engine substantially simultaneously with theprinting by the printhead of the first print engine;

a first communications link interconnecting the first print controllerand the second print controller for synchronizing the print controllers;and

a second communications link interconnecting at least one of the printcontrollers with a host system for receipt from the host system ofdescriptions of pages to be printed on said surfaces of the sheet ofprint media by the print engines.

In this specification, unless the context clearly indicate otherwise theterm “page width printhead”, is to be understood as a printhead having aprinting zone that prints one line at a time on a page, the line beingparallel either to a longer edge or a shorter edge of the page. The lineis printed as a whole as the page moves past the printhead and theprinthead is stationary, i.e. it does not raster.

The first print controller is, preferably, a master print controllerwith the second print controller being a slave print controller operableunder command of the master print controller on receipt of signals viathe first communications link.

The first communications link may be a bi-directional link enabling thetransmission of data from the slave print controller to the master printcontroller. Thus, after the printing of a page, or more frequently, themaster print controller may obtain ink consumption information from theslave print controller via the first communications link. The masterprint controller uses this to update the remaining ink volume in an inkcartridge. Further, the master print controller and slave printcontroller may also exchange error events and host-initiated printerreset commands via the first communications link.

The master print controller may have the second communications linkconnected to it to present a unified view to the host system to mask thepresence of the slave print controller. The master print controller mayprint described pages on a rear surface of the print media with theslave print controller printing on a front surface of the print media sothat the master print controller always has a page buffer available fora page description destined for the slave print controller.

Print synchronization may be achieved by the master print controllercontrolling a printing operation of the slave print controller.Printhead interfaces of both print controllers may be synchronized to ashared line synchronization signal generated by one of the printcontrollers.

According to a second aspect of the invention there is provided a methodof controlling printing on both surfaces of a sheet of print media, themethod including the steps of:

receiving data relating to a first page to be printed in a first printcontroller of a first print engine;

transmitting the data relating to the first page from the first printcontroller to a second print controller of a second print engine;

receiving data relating to a second page to be printed in the firstprint controller; and

controlling printing by the print engines under command of the firstprint controller to achieve synchronization of printing of the pages bythe first print controller and the second print controller on rear andfront surfaces of the print media, respectively.

The method may include synchronizing printhead interfaces of both printcontrollers by means of a shared line synchronization signal generatedby the first print controller which is a master print controller.Further, the method may include transmitting data relating to the pagesto be printed from a host system to the master print controller by meansof a host communications link, the master print controller determiningwhether or not said data are to be routed to the second print controllerwhich is a slave print controller.

The method may also include receiving said data in its entirety in amemory of the master print controller before forwarding it to the slaveprint controller.

The method may include selecting the master print controller to printthe rear surface of the print media to ensure that the master printcontroller always has a page buffer available for a page descriptiondestined for the slave print controller.

As described above, the method may include, periodically, transmittingpredetermined data from the slave print controller to the master printcontroller.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described by way of example with reference to theaccompanying drawings in which:—

FIG. 1 shows a front view of a CePrint printer, in accordance with afirst embodiment if the invention;

FIG. 2 shows a front view of the printer, in accordance with a secondembodiment of the invention;

FIG. 3 shows a side view of the printer of FIG. 1;

FIG. 4 shows a plan view of the printer of FIG. 1;

FIG. 5 shows a side view of the printer of FIG. 2;

FIG. 6 shows a table illustrating a sustained printing rate of theprinter achievable with double-buffering in the printer;

FIG. 7 shows a flowchart illustrating the conceptual data flow fromapplication to printed page;

FIG. 8 shows a schematic, sectional plan view of the printer;

FIG. 8A shows a detailed view of the area circled in FIG. 8;

FIG. 9 shows a side view, from one side, of the printer of FIG. 8;

FIG. 10 shows a side view, from the other side, of the printer of FIG.8;

FIG. 11 shows a schematic side view of part of the printer showing therelationship between a print engine and image transfer mechanism of theprinter;

FIG. 12A shows a schematic side view of part of the devices of FIG. 111showing the printhead in a parked, non-printing condition relative tothe transfer mechanism;

FIG. 12B shows a schematic side view of the part of the devices of FIG.11 showing the printhead in a printing condition relative to thetransfer mechanism;

FIG. 13 shows a schematic side view of the arrangement of the printheadsand transfer mechanisms of the double-sided printer of FIG. 2;

FIG. 14 shows a three-dimensional, exploded view of a paper drive chainof the printer;

FIG. 15 shows a three-dimensional view of the printing system of theprinter;

FIG. 16 shows an enlarged, three-dimensional view of part of theprinting system of FIG. 15;

FIG. 17 shows a simple encoding sample;

FIG. 18 shows a block diagram of printer controller architecture;

FIG. 19 shows a flowchart of page expansion and printing data flow;

FIG. 20 shows a block diagram of an EDRL expander unit;

FIG. 21 shows a block diagram of an EDRL stream decoder;

FIG. 22 shows a block diagram of a runlength decoder;

FIG. 23 shows a block diagram of a runlength encoder;

FIG. 24 shows a block diagram of a JPEG decoder;

FIG. 25 shows a block diagram of a halftoner/compositor unit;

FIG. 26 shows a diagram of the relationship between page widths andmargins;

FIG. 27 shows a block diagram of a multi-threshold dither unit;

FIG. 28 shows a block diagram of logic of the triple-threshold dither;

FIG. 29 shows a block diagram of a speaker interface;

FIG. 30 shows a block diagram of a dual printer controllerconfiguration;

FIG. 31 shows a schematic representation of a Memjet page widthprinthead;

FIG. 32 shows a schematic representation of a pod of twelve printingnozzles numbered in firing order;

FIG. 33 shows a schematic representation of the same pod with thenozzles numbered in load order;

FIG. 34 shows a schematic representation of a chromapod comprising onepod of each color;

FIG. 35 shows a schematic representation of a podgroup comprising fivechromapods;

FIG. 36 shows a schematic representation of a phasegroup comprising twopodgroups;

FIG. 37 shows a schematic representation of the relationship betweensegments, firegroups, phasegroups, podgroups and chromapods:

FIG. 38 shows a phase diagram of AEnable and BEnable lines during atypical printing cycle;

FIG. 39 shows a block diagram of a printhead interface;

FIG. 40 shows a block diagram of a Memjet interface;

FIG. 41 shows a flow diagram of the generation of AEnable and BEnablepulse widths;

FIG. 42 shows a block diagram of dot count logic;

FIG. 43 shows a conceptual overview of double buffering during printingof lines N and N+1;

FIG. 44 shows a block diagram of the structure of a line loader/formatunit;

FIG. 45 shows a conceptual structure of a buffer;

FIG. 46 shows a block diagram of the logical structure of a buffer;

FIG. 47 shows a diagram of the structure and size of a two-layer pagebuffer; and

FIG. 48 shows a block diagram of a Windows 9x/NT/CE printing system withprinter driver components indicated.

DETAILED DESCRIPTION OF THE DRAWINGS 1 Introduction

The printer, in accordance with the invention, is a high-performancecolor printer which combines photographic-quality image reproductionwith magazine-quality text reproduction. It utilizes an 8″ page-widthmicroelectromechanical inkjet printhead which produces 1600 dots perinch (dpi) bi-level CMYK (Cyan, Magenta, Yellow, blacK). In thisdescription the printhead technology shall be referred to as “Memjet”,and the printer shall be referred to as “CePrint”.

CePrint is conceived as an original equipment manufacture (OEM) partdesigned for inclusion primarily in consumer electronics (CE) devices.Intended markets include televisions, VCRs, PhotoCD players, DVDplayers, Hi-fi systems, Web/Internet terminals, computer monitors, andvehicle consoles. As will be described in greater detail below, itfeatures a low-profile front panel and provides user access to paper andink via an ejecting tray. It operates in a horizontal orientation underdomestic environmental conditions.

CePrint exists in single- and double-sided versions. The single-sidedversion prints 30 full-color A4 or Letter pages per minute. Thedouble-sided version prints 60 full-color pages per minute (i.e. 30sheets per minute). Although CePrint supports both paper sizes, it isconfigured at time of manufacture for a specific paper size.

1.1 Operational Overview

CePrint reproduces black text and graphics directly using bi-levelblack, and continuous-tone (contone) images and graphics using ditheredbi-level CMYK. For practical purposes, CePrint supports a blackresolution of 800 dpi, and a contone resolution of 267 pixels per inch(ppi).

CePrint is embedded in a CE device, and communicates with the CE device(host) processor via a relatively low-speed (1.5 MBytes/s) connection.CePrint relies on the host processor to render each page to the level ofcontone pixels and black dots. The host processor compresses eachrendered page to less than 3 MB for sub-two-second delivery to theprinter. CePrint decompresses and prints the page line by line at thespeed of its micro-electromechanical inkjet (Memjet) printhead. CePrintcontains sufficient buffer memory for two compressed pages (6 MB),allowing it to print one page while receiving the next, but does notcontain sufficient buffer memory for even a single uncompressed page(119 MB).

The double-sided version of CePrint contains two printheads whichoperate in parallel. These printheads are fed by separate data paths,each of which replicates the logic found in the single-sided version ofCePrint. The double-sided version has a correspondingly fasterconnection to the host processor (3 MB/s).

2 Product Specification

Table 1 gives a summary product specification of the single-sided anddouble-sided versions of the CePrint unit.

TABLE 1 CePrint Specification single-sided version double-sided versiondot pitch 1600 dpi paper standard A4/Letter paper tray capacity 150sheets print speed 30 pages per minute 60 pages per minute, 30 sheetsper minute warm-up time nil first print time 2–6 seconds subsequentprints 2 seconds/sheet color model 32-bit CMYK process color printablearea full page (full edge bleed) printhead page width Memjet with dualprintheads 54,400 nozzles print method self-cleaning transfer roller,dual transfer rollers titanium nitride (TiN) coated size (h × w × d) 40mm × 272 mm × 60 mm × 272 mm × 416 mm 416 mm weight 3 kg (approx.) 4 kg(approx.) power supply 5V, 4A 5V, 8A ink cartridge 650 pages at 15%coverage color capacity ink cartridge 900 pages at 15% coverage blackcapacity ink cartridge size 21 mm × 188 mm × 38 mm (h × w × d)

3 Memjet-Based Printing

A Memjet printhead produces 1600 dpi bi-level CMYK. On low-diffusionpaper, each ejected drop forms an almost perfectly circular 22.5 mmdiameter dot. Dots are easily produced in isolation, allowingdispersed-dot dithering to be exploited to its fullest. Since the Memjetprinthead is page-width and operates with a constant paper velocity, thefour color planes are printed in perfect registration, allowing idealdot-on-dot printing. Since there is consequently no spatial interactionbetween color planes, the same dither matrix is used for each colorplane.

A page layout may contain a mixture of images, graphics and text.Continuous-tone (contone) images and graphics are reproduced using astochastic dispersed-dot dither. Unlike a clustered-dot (oramplitude-modulated) dither, a dispersed-dot (or frequency-modulated)dither reproduces high spatial frequencies (i.e. image detail) almost tothe limits of the dot resolution, while simultaneously reproducing lowerspatial frequencies to their full color depth, when spatially integratedby the eye. A stochastic dither matrix is carefully designed to be freeof objectionable low-frequency patterns when tiled across the image. Assuch its size typically exceeds the minimum size required to support anumber of intensity levels (i.e. 16×16×8 bits for 257 intensity levels).CePrint uses a dither volume of size 64×64×3×8 bits. The volume providesan extra degree of freedom during the design of the dither by allowing adot to change states multiple times through the intensity range, ratherthan just once as in a conventional dither matrix.

Human contrast sensitivity peaks at a spatial frequency of about 3cycles per degree of visual field and then falls off logarithmically,decreasing by a factor of 100 beyond about 40 cycles per degree andbecoming immeasurable beyond 60 cycles per degree. At a normal viewingdistance of 12 inches (about 300 mm), this translates roughly to 200-300cycles per inch (cpi) on the printed page, or 400-600 samples per inchaccording to Nyquist's theorem. Contone resolution beyond about 400pixels per inch (ppi) is therefore of limited utility, and in factcontributes slightly to color error through the dither.

Black text and graphics are reproduced directly using bi-level blackdots, and are therefore not antialiased (i.e. low-pass filtered) beforebeing printed. Text is therefore supersampled beyond the perceptuallimits discussed above, to produce smoother edges when spatiallyintegrated by the eye. Text resolution up to about 1200 dpi continues tocontribute to perceived text sharpness (assuming low-diffusion paper, ofcourse).

4 Page Delivery Architecture

4.1 Page Image Sizes

CePrint prints A4 and Letter pages with full edge bleed. Correspondingpage image sizes are set out in Table 2 for various spatial resolutionsand color depths used in the following discussion. Note that the size ofan A4 page exceeds the size of a Letter page, although the Letter pageis wider. Page buffer requirements are therefore based on A4, while linebuffer requirements are based on Letter.

TABLE 2 Page Image Sizes spatial resolution color depth A4^(a) pageLetter^(b) page (pixels/inch) (bits/pixel) buffer size buffer size 160032 948 MB 913 MB 800 32 237 MB 228 MB 400 32 59.3 MB 57.1 MB 267 32 26.4MB 25.4 MB 1600 4 119 MB 114 MB 800 4 29.6 MB 28.5 MB 1600 1 29.6 MB28.5 MB 800 1 7.4 MB 7.1 MB ^(a)210 mm × 297 mm, or 8.3″ × 11.7″^(b)8.5″ × 11″4.2 Constraints

The act of interrupting a Memjet-based printer during the printing of apage produces a visible discontinuity, so it is advantageous for theprinter to receive the entire page before commencing printing, toeliminate the possibility of buffer underrun. Furthermore, if thetransmission of the page from the host to the printer takes significanttime in relation to the time it takes to print the page, then it isadvantageous to provide two page buffers in the printer so that one pagecan be printed while the next is being received. If the transmissiontime of a page is less than its 2-second printing time, thendouble-buffering allows the full 30 pages/minute page rate of CePrint tobe achieved.

FIG. 6 illustrates the sustained printing rate achievable withdouble-buffering in the printer, assuming 2-second page rendering and2-second page transmission.

Assuming it is economic for the printer to have a maximum of only 8 MBof memory (i.e. a single 64 Mbit DRAM), then less than 4 MB is availablefor each page buffer in the printer, imposing a limit of less than 4 MBon the size of the page image. To allow for program and working memoryin the printer, we limit this to 3 MB per page image.

Assuming the printer has only a typical low-speed connection to the hostprocessor, then the speed of this connection is 1-2 MB/s (i.e. 2 MB/sfor parallel port, 1.5 MB/s for USB, and 1 MB/s for 10Base-T Ethernet).Assuming 2-second page transmission (i.e. equal to the printing time),this imposes a limit of 2-4 MB on the size of the page image, i.e. alimit similar to that imposed by the size of the page buffer.

In practice, because the host processor and the printer can be closelycoupled, a high-speed connection between them may be feasible.

Whatever the speed of the host connection required by the single-sidedversion of CePrint, the double-sided version requires a connection oftwice that speed.

4.3 Page Rendering and Compression

Page rendering (or rasterization) can be split between the hostprocessor and printer in various ways. Some printers support a full pagedescription language (PDL) such as Postscript, and containcorrespondingly sophisticated renderers. Other printers provide specialsupport only for rendering text, to achieve high text resolution. Thisusually includes support for built-in or downloadable fonts. In eachcase the use of an embedded renderer reduces the rendering burden on thehost processor and reduces the amount of data transmitted from the hostprocessor to the printer. However, this comes at a price. These printersare more complex than they might be, and are often unable to providefull support for the graphics system of the host, through whichapplication programs construct, render and print pages. They fail toexploit the possibly high performance of the host processor.

CePrint relies on the host processor to render pages, i.e. contoneimages and graphics to the pixel level, and black text and graphics tothe dot level. CePrint contains only a simple rendering engine whichdithers the contone data and combines the results with any foregroundbi-level black text and graphics. This strategy keeps the printersimple, and independent of any page description language or graphicssystem. It fully exploits the high performance expected in the hostprocessor of a multimedia CE device. The downside of this strategy isthe potentially large amount of data which must be transmitted from thehost processor to the printer. We therefore use compression to reducethe page image size to the 3 MB required to allow a sustained printingrate of 30 pages/minute.

An 8.3″×11.7″ A4 page has a bi-level CMYK page image size of 119 MBytesat 1600 dpi, and a contone CMYK pagesize of 59.3 MB at 400 ppi.

We use JPEG compression to compress the contone data. Although JPEG isinherently lossy, for compression ratios of 10:1 or less the loss isusually negligible. To achieve a high-quality compression ratio of lessthan 10:1, and to obtain an integral contone to bi-level ratio, wechoose a contone resolution of 267 ppi. This yields a contone CMYKpagesize of 25.5 MB, a corresponding compression ratio of 8.5:1 to fitwithin the 3 MB/page limit, and a contone to bi-level ratio of 1:6 ineach dimension.

A full page of black text (and/or graphics) rasterized at printerresolution (1600 dpi) yields a bi-level image of 29.6 MB. Sincerasterizing text at 1600 dpi places a heavy burden on the host processorfor a small gain in quality, we choose to rasterize text at 800 dpi.This yields a bi-level image of 7.4 MB, requiring a lossless compressionratio of less than 2.5:1 to fit within the 3 MB/page limit. We achievethis using a two-dimensional bi-level compression scheme similar to thecompression scheme used in Group 4 Facsimile.

As long as the image and text regions of a page are non-overlapping, anycombination of the two fits within the 3 MB limit. If text lies on topof a background image, then the worst case is a compressed page imagesize approaching 6 MB (depending on the actual text compression ratio).This fits within the printer's page buffer memory, but preventsdouble-buffering of pages in the printer, thereby reducing the printer'spage rate by two-thirds, i.e. to 10 pages/minute.

4.4 Page Expansion and Printing

As described above, the host processor renders contone images andgraphics to the pixel level, and black text and graphics to the dotlevel. These are compressed by different means and transmitted togetherto the printer.

The printer contains two 3 MB page buffers—one for the page beingreceived from the host, and one for the page being printed. The printerexpands the compressed page as it is being printed. This expansionconsists of decompressing the 267 ppi contone CMYK image data,halftoning the resulting contone pixels to 1600 dpi bi-level CMYK dots,decompressing the 800 dpi bi-level black text data, and compositing theresulting bi-level black text dots over the corresponding bi-level CMYKimage dots.

The conceptual data flow from the application to the printed page isillustrated in FIG. 7.

5 Printer Hardware

CePrint is conceived as an OEM part designed for inclusion primarily inconsumer electronics (CE) devices. Intended markets include televisions,VCRs, PhotoCD players, DVD players, Hi-fi systems, Web/Internetterminals, computer monitors, and vehicle consoles. It features alow-profile front panel and provides user access to paper and ink via anejecting tray. It operates in a horizontal orientation under domesticenvironmental conditions.

Because of the simplicity of the page width Memjet printhead, CePrintcontains an ultra-compact print mechanism which yields an overallproduct height of 40 mm for the single-sided version and 60 mm for thedouble-sided version.

The nature of an OEM product dictates that it should be simple in styleand reflect minimum dimensions for inclusion into host products. CePrintis styled to be accommodated into all of the target market products andhas minimum overall dimensions of 40 mm high×272 mm wide×416 mm deep.The only cosmetic part of the product is the front fascia and front trayplastics. These can be re-styled by a manufacturer if they wish to blendCePrint with a certain piece of equipment.

Front views of the two versions of CePrint or the printer areillustrated in FIGS. 1 and 2, respectively, of the drawings and aredesignated generally by the reference numeral 10. It is to be notedthat, mechanically, both versions are the same, apart from the greaterheight of the double-sided version to accommodate a second print engineinstead of a pinch roller. This will be described in greater detailbelow. Side and plan views of the single-sided version of CePrint areillustrated in FIGS. 3 and 4 respectively. The side view of thedouble-sided version of CePrint is illustrated in FIG. 5.

CePrint 10 is a motorized A4/Letter paper tray with a removable inkcartridge and a Memjet printhead mechanism. It includes a front panel 12housing a paper eject button 14, a power LED 16, an out-of-ink LED 18and an out-of-paper LED 20. A paper tray 22 is slidably arrangedrelative to the front panel 12. When the paper tray 22 is in its “home”position, a paper output slot 24 is defined between the front panel 12and the paper tray 22.

The front panel 12 fronts a housing 26 containing the working parts ofthe printer 10. As illustrated in 5 of the drawings, the housing 26, inthe case of the double-sided version is stepped, at 28, towards thefront panel 12 (FIGS. 1 and 2) to accommodate the second print engine.The housing 26 covers a metal chassis 30 (FIG. 8), fascias, the (molded)paper tray 22, an ink cartridge 32 (FIG. 10), three motors, a flex PCB34 (FIG. 8A), a rigid PCB 36 (FIG. 8) and various injection moldings andsmaller parts to achieve a cost effective high volume product.

The printer 10 is simple to operate, only requiring user interactionwhen paper or ink need replenishing as indicated by the front-panel LEDs20 or 18, respectively. The paper handling mechanisms are similar tocurrent printer applications and are therefore considered reliable. Inthe rare event of a paper jam, the action of ejecting the paper trayallows the user to deal with the problem. The tray 22 has a sensor whichretracts the tray if the tray 22 is pushed. If the tray 22 jams on theway in, this is also sensed and the tray 22 is re-ejected. This allowsthe user to be lazy in operating the tray 22 by just pushing it to closeand protects the unit from damage should the tray 22 get knocked whilein the out position. It also caters for children sticking fingers in thetray 22 while closing. Ink is replaced by inserting a new cartridge 32into the paper tray 22 (FIG. 8) and securing it by a cam lock levermechanism.

5.1 Overview

The overall views of CePrint 10 are shown in FIGS. 8 to 10. As shown inFIG. 9 the chassis 30 includes base metalwork 38 on which front rollerwheels 40 of the paper tray 22 are slidable. A bracket 42 accommodatesmotors 44, 46 and 48 and gears 50 and 52 for ejecting the paper tray 22and driving a paper pick-up roller 54.

Attached to the bracket 42 and the base metalwork 38 are two guide rails56 that allow the molded paper tray 22 and its rear roller wheels 58 toslide forward. As described above, the tray 22 also sits on frontrollers 40 and this provides a strong, low friction and steady method ofejection and retraction. The flex PCB 34 (FIG. 8A) runs from the mainPCB 36 via the motors 44, 46 and 48 to a contact molding 60 and alightpipe area 62. An optical sensor on the flex PCB 34 allows the trayejector motor 46 to retract the tray 22 independently of the ejectbutton 14 if the tray 22 is pushed when in the out position by sensing ahole in a gear wheel 64 (FIG. 9). Similarly, the tray 22 is ejected ifthere is any stoppage during retraction.

The contact molding 60 has a foam pad 66 that the flex PCB 34 is fixedonto and provides data and power contacts to the printhead and bus barsduring printing.

A transfer roller 68 (FIG. 11) has two end caps 69 (FIG. 8A) that run inlow friction bearing assemblies 70. One of the end caps 69 has aninternal gear that acts on a small gear 72 (FIG. 14) which transferspower through a reduction gear 74 to a worm drive. This is reducedfurther via another gear to a motor worm drive 76 mounted on an outputshaft of a stepper motor 124 arranged within the transfer roller 68.This solution for a motor drive assembly that is housed inside thetransfer roller 68 saves space for future designs and mounts onto asmall chassis 78 (FIG. 8A) that is attached to ink connector moldings80, 82.

An ink connector 84 has four pins 86 with an ejector plate 88 andsprings 90 that interfaces with the ink cartridge 32 (FIG. 10). The inkcartridge 32 is accessed via a cam lock lever and spring 92 (FIG. 8).The ink is conducted through molded channels into a flexiblefour-channel hose connector 94 that interfaces with a printheadcartridge end cap 96. The other end of the printhead cartridge has adifferent flexible sealing connector 98 (FIG. 8) on the end cap to allowink to be drawn through the cartridge during assembly and effectivelycharge the unit and ink connector with ink in a sealed environment. Theprinthead and ink connector assemblies are mounted directly into thepaper tray 22.

The paper tray 22 has several standard paper handling components, namelya metal base channel 100 (FIG. 8) with low friction pads 102, sprung bytwo compression springs 104 and two metal paper guides 106 with arms 108secured by rivets. The paper is aligned to one side of the tray 22 by aspring steel clip 110. The tray 22 is normally configured to take A4paper, but Letter size paper is accommodated by relocating one of thepaper guide assemblies 106 and clipping a plate into the paper tray 22to provide a rear stop. The paper tray 22 can accommodate up to 150sheets.

As is standard practice for adding paper, the metal base channel 100 ispushed down and is latched using a tray lock molding 112 and a returnspring 114. When paper has been added and the tray 22 retracted, thetray lock molding 112 is unlatched by hitting a metal return 116 in thebase metalwork 38 (FIG. 9).

The printer 10 is now ready to print. When activated, the paper pick-uproller 54 is driven by a small drive gear 118 that meshes with anotherdrive gear 50 and a normal motor 44. The roller 54 is located to thebase metalwork 38 (FIG. 9) by two heat staked retainer moldings 120(FIG. 8). A small molding on the end of the pick-up roller 54 acts witha sensor on the flex PCB 34 to accurately position the pick-up roller 54in a parked position so that paper and tray 22 can be withdrawn withouttouching it when ejecting. This accurate positioning also allows theroller 54 to feed the sheet to the transfer roller 68 (FIG. 10) with afixed number of revolutions. As the transfer roller 68 is running at asimilar speed there should be no problem with take-up of the paper. Anoptical sensor 122 mounted into the housing 26 finds the start of eachsheet and engages a transfer motor 124 (FIG. 14), so there is no problemif for example a sheet has moved forward of the roller during anystrange operations.

The main PCB 36 is mounted onto the base metalwork 38 via standard PCBstandoffs 126 and is fitted with a data connector 128 and a DC connector130.

The front panel 12 is mounted onto the base metalwork 38 using snapdetails and a top metal cover 132 completes the overall product withRFI/EMI integrity via four fixings 134.

5.2 Printhead Assembly and Image Transfer Mechanism

The print engine is shown in greater detail in FIG. 11 and is designatedgenerally by the reference numeral 140. The Memjet printhead assemblyis, in turn, designated by the reference numeral 142. This representsone of the four possible ways to deploy the Memjet printhead 143 inconjunction with the ink cartridge 32 in a product such as CePrint 10:

permanent printhead, replaceable ink cartridge (as shown in FIG. 11)

separate replaceable printhead cartridge and ink cartridge

refillable combined printhead and ink cartridge

disposable combined printhead and ink cartridge

The Memjet printhead 143 prints onto the titanium nitride (TiN) coatedtransfer roller 68 which rotates in an anticlockwise direction totransfer the image onto a sheet of paper 144. The paper 144 is pressedagainst the transfer roller 68 by a spring-loaded rubber coated pinchroller 145 in the case of the single-sided version. As illustrated inFIG. 13, in the case of the double-sided version, the paper 144 ispressed against one of the transfer rollers 68 under the action of theopposite transfer roller 68. After transferring the image to the paper144 the transfer roller 68 continues past a cleaning sponge 146 andfinally a rubber wiper 148. The sponge 146 and the wiper 148 form acleaning station for cleaning the surface of the transfer roller 68.

While operational, the printhead assembly 142 is held off the transferroller 68 by a solenoid 150 as shown in FIG. 12B. When not operational,the printhead assembly 142 is parked against the transfer roller 68 asshown in FIG. 12A. The printhead's integral elastomeric seal 152 sealsthe printhead assembly 142 and prevents the Memjet printhead 143 fromdrying out.

In the double-sided version of CePrint 10, there are dual print engines140, each with its associated printhead assembly 142 and transfer roller68, mounted in opposition as illustrated in FIG. 13. The lower printengine 140 is fixed while the upper print engine 140 pivots and issprung to press against the paper 144. As previously described, theupper transfer roller 68 takes the place of the pinch roller 145 in thesingle-sided version.

The relationship between the ink cartridge 32, printhead assembly 142and the transfer roller 68 is shown in greater detail in FIGS. 15 and 16of the drawings. The ink cartridge has four reservoirs 154, 156, 158 and160 for cyan, magenta, yellow and black ink respectively.

Each reservoir 154-160 is in flow communication with a correspondingreservoir 164-170 in the printhead assembly 142. These reservoirs, inturn, supply ink to a Memjet printhead chip 143 (FIG. 16) via an inkfilter 174. It is to be noted in FIG. 16 that the elastomeric cappingseal 152 is arranged on both sides of the printhead chip 143 to assistin sealing when the printhead assembly 142 is parked against thetransfer roller 68.

Power is supplied to the solenoid via busbars 176.

6 Printer Control Protocol

This section describes the printer control protocol used between a hostand CePrint 10. It includes control and status handling as well as theactual page description.

6.1 Control and Status

The printer control protocol defines the format and meaning of messagesexchanged by the host processor and the printer 10. The control protocolis defined independently of the transport protocol between the hostprocessor and the printer 10, since the transport protocol depends onthe exact nature of the connection.

Each message consists of a 16-bit message code, followed bymessage-specific data which may be of fixed or variable size.

All integers contained in messages are encoded in big-endian byte order.

Table 3 defines command messages sent by the host processor to theprinter 10.

TABLE 3 Printer command messages command message message codedescription reset printer 1 Resets the printer to an idle state (i.e.ready and not printing). get printer status 2 Gets the current printerstatus. start document 3 Starts a new document. start page 4 Starts thedescription of a new output page. page band 5 Describes a band of thecurrent output page. end page 6 Ends the description of the currentoutput page. end document 7 Ends the current document.

The reset printer command can be used to reset the printer to clear anerror condition, and to abort printing.

The start document command is used to indicate the start of a newdocument. This resets the printer's page count, which is used in thedouble-sided version to identify odd and even pages. The end documentcommand is simply used to indicate the end of the document.

The description of an output page consists of a page header whichdescribes the size and resolution of the page, followed by one or morepage bands which describe the actual page content. The page header istransmitted to the printer in the start page command. Each page band istransmitted to the printer in a page band command. The last page band isfollowed by an end page command. The page description is described indetail in Table 4.2.

Table 4 defines response messages sent by the printer to the hostprocessor.

TABLE 4 Printer response messages response message message codedescription printer status 8 Contains the current printer status (asdefined in Table 7). page error 9 Contains the most recent page errorcode (as defined in Table 8).

A printer status message is normally sent in response to a get printerstatus command. However, the nature of the connection between the hostprocessor and the printer may allow the printer to send unsolicitedstatus messages to the host processor. Unsolicited status messages allowtimely reporting of printer exceptions to the host processor, andthereby to the user, without requiring the host processor to poll theprinter on a frequent basis.

A page error message is sent in response to each start page, page bandand end page command.

Table 5 defines the format of the 16-bit printer status contained in theprinter status message.

TABLE 5 Printer status format field bit description ready 0 The printeris ready to receive a page. printing 1 The printer is printing. error 2The printer is in an error state. paper tray missing 3 The paper tray ismissing. paper tray empty 4 The paper tray is empty. ink cartridgemissing 5 The ink cartridge is missing. ink cartridge empty 6 The inkcartridge is empty. ink cartridge error 7 The ink cartridge is in anerror state. (reserved) 8–15 Reserved for future use.

Table 6 defines page error codes which may be returned in a page errormessage.

TABLE 6 Page error codes error code value description no error 0 Noerror. bad signature 1 The signature is not recognized. bad version 2The version is not supported. bad parameter 3 A parameter is incorrect.6.2 Page Description

CePrint 10 reproduces black at full dot resolution (1600 dpi), butreproduces contone color at a somewhat lower resolution usinghalftoning. The page description is therefore divided into a black layerand a contone layer. The black layer is defined to composite over thecontone layer.

The black layer consists of a bitmap containing a 1-bit opacity for eachpixel. This black layer matte has a resolution which is an integerfactor of the printer's dot resolution. The highest supported resolutionis 1600 dpi, i.e. the printer's full dot resolution.

The contone layer consists of a bitmap containing a 32-bit CMYK colorfor each pixel. This contone image has a resolution which is an integerfactor of the printer's dot resolution. The highest supported resolutionis 267 ppi, i.e. one-sixth the printer's dot resolution.

The contone resolution is also typically an integer factor of the blackresolution, to simplify calculations in the printer driver. This is nota requirement, however.

The black layer and the contone layer are both in compressed form forefficient transmission over the low-speed connection to the printer.

6.2.1 Page Structure

CePrint prints with full edge bleed using an 8.5″ printhead. It imposesno margins and so has a printable page area which corresponds to thesize of its paper (A4 or Letter).

The target page size is constrained by the printable page area, less theexplicit (target) left and top margins specified in the pagedescription.

6.2.2 Page Description Format

Apart from being implicitly defined in relation to the printable pagearea, each page description is complete and self-contained. There is nodata transmitted to the printer separately from the page description towhich the page description refers.

The page description consists of a page header which describes the sizeand resolution of the page, followed by one or more page bands whichdescribe the actual page content.

Table 7 shows the format of the page header.

TABLE 7 Page header format field format description signature 16-bitinteger Page header format signature. version 16-bit integer Page headerformat version number. structure size 16-bit integer Size of pageheader. target resolution (dpi) 16-bit integer Resolution of targetpage. This is always 1600 for CePrint. target page width 16-bit integerWidth of target page, in dots. target page height 16-bit integer Heightof target page, in dots. target left margin 16-bit integer Width oftarget left margin, in dots. target top margin 16-bit integer Height oftarget top margin, in dots. black scale factor 16-bit integer Scalefactor from black resolution to target resolution (must be 2 orgreater). black page width 16-bit integer Width of black page, in blackpixels. black page height 16-bit integer Height of black page, in blackpixels. contone scale factor 16-bit integer Scale factor from contoneresolution to target resolution (must be 6 or greater). contone pagewidth 16-bit integer Width of contone page, in contone pixels. contonepage height 16-bit integer Height of contone page, in contone pixels.

The page header contains a signature and version which allow the printerto identify the page header format. If the signature and/or version aremissing or incompatible with the printer, then the printer can rejectthe page.

The page header defines the resolution and size of the target page. Theblack and contone layers are clipped to the target page if necessary.This happens whenever the black or contone scale factors are not factorsof the target page width or height.

The target left and top margins define the positioning of the targetpage within the printable page area.

The black layer parameters define the pixel size of the black layer, andits integer scale factor to the target resolution.

The contone layer parameters define the pixel size of the contone layer,and its integer scale factor to the target resolution.

Table 8 shows the format of the page band header.

TABLE 8 Page band header format field format description signature16-bit integer Page band header format signature. version 16-bit integerPage band header format version number. structure size 16-bit integerSize of page band header. black band height 16-bit integer Height ofblack band, in black pixels. black band data size 32-bit integer Size ofblack band data, in bytes. contone band height 16-bit integer Height ofcontone band, in contone pixels. contone band data 32-bit integer Sizeof contone band data, in size bytes.

The black layer parameters define the height of the black band, and thesize of its compressed band data. The variable-size black band datafollows the fixed-size parts of the page band header.

The contone layer parameters define the height of the contone band, andthe size of its compressed page data. The variable-size contone banddata follows the variable-size black band data.

Table 9 shows the format of the variable-size compressed band data whichfollows the page band header.

TABLE 9 Page band data format field format description black band dataEDRL bytestream Compressed hi-level black band data. contone band dataJPEG bytestream Compressed contone CMYK band data.

The variable-size black band data and the variable-size contone banddata are aligned to 8-byte boundaries. The size of the required paddingis included in the size of the fixed-size part of the page band headerstructure and the variable-size black band data.

The entire page description has a target size of less than 3 MB, and amaximum size of 6 MB, in accordance with page buffer memory in theprinter.

The following sections describe the format of the compressed black layerand the compressed contone layer.

6.2.3 Bi-Level Black Layer Compression

6.2.3.1 Group 3 and 4 Facsimile Compression

The Group 3 Facsimile compression algorithm losslessly compressesbi-level data for transmission over slow and noisy telephone lines. Thebi-level data represents scanned black text and graphics on a whitebackground, and the algorithm is tuned for this class of images (it isexplicitly not tuned, for example, for halftoned bi-level images). The1D Group 3 algorithm runlength-encodes each scanline and thenHuffman-encodes the resulting runlengths. Runlengths in the range 0 to63 are coded with terminating codes. Runlengths in the range 64 to 2623are coded with make-up codes, each representing a multiple of 64,followed by a terminating code. Runlengths exceeding 2623 are coded withmultiple make-up codes followed by a terminating code. The Huffmantables are fixed, but are separately tuned for black and white runs(except for make-up codes above 1728, which are common). When possible,the 2D Group 3 algorithm encodes a scanline as a set of short edgedeltas (0, ±1, ±2, ±3) with reference to the previous scanline. Thedelta symbols are entropy-encoded (so that the zero delta symbol is onlyone bit long etc.) Edges within a 2D-encoded line which can't bedelta-encoded are runlength-encoded, and are identified by a prefix. 1D-and 2D-encoded lines are marked differently. 1D-encoded lines aregenerated at regular intervals, whether actually required or not, toensure that the decoder can recover from line noise with minimal imagedegradation. 2D Group 3 achieves compression ratios of up to 6:1.

The Group 4 Facsimile algorithm losslessly compresses bi-level data fortransmission over error-free communications lines (i.e. the lines aretruly error-free, or error-correction is done at a lower protocollevel). The Group 4 algorithm is based on the 2D Group 3 algorithm, withthe essential modification that since transmission is assumed to beerror-free, 1D-encoded lines are no longer generated at regularintervals as an aid to error-recovery. Group 4 achieves compressionratios ranging from 20:1 to 60:1 for the CCITT set of test images.

The design goals and performance of the Group 4 compression algorithmqualify it as a compression algorithm for the bi-level black layer.However, its Huffman tables are tuned to a lower scanning resolution(100-400 dpi), and it encodes runlengths exceeding 2623 awkwardly. At800 dpi, our maximum runlength is currently 6400. Although a Group 4decoder core might be available for use in the printer controller chip(Section 7), it might not handle runlengths exceeding those normallyencountered in 400 dpi facsimile applications, and so would requiremodification.

Since most of the benefit of Group 4 comes from the delta-encoding, asimpler algorithm based on delta-encoding alone is likely to meet ourrequirements. This approach is described in detail below.

6.2.3.2 Bi-Level Edge Delta and Runlength (EDRL) Compression Format

The edge delta and runlength (EDRL) compression format is based looselyon the Group 4 compression format and its precursors.

EDRL uses three kinds of symbols, appropriately entropy-coded. These arecreate edge, kill edge, and edge delta. Each line is coded withreference to its predecessor. The predecessor of the first line isdefined to a line of white. Each line is defined to start off white. Ifa line actually starts of black (the less likely situation), then itmust define a black edge at offset zero. Each line must define an edgeat its left-hand end, i.e. at offset page width.

An edge can be coded with reference to an edge in the previous line ifthere is an edge within the maximum delta range with the same sense(white-to-black or black-to-white). This uses one of the edge deltacodes. The shorter and likelier deltas have the shorter codes. Themaximum delta range (±2) is chosen to match the distribution of deltasfor typical glyph edges. This distribution is mostly independent ofpoint size. A typical example is given in Table 10.

TABLE 10 Edge delta distribution for 10 point Times at 800 dpi |delta|probability 0 65% 1 23% 2  7% ≧3   5%

An edge can also be coded using the length of the run from the previousedge in the same line. This uses one of the create edge codes for short(7-bit) and long (13-bit) runlengths. For simplicity, and unlike Group4, runlengths are not entropy-coded. In order to keep edge deltasimplicitly synchronized with edges in the previous line, each unusededge in the previous line is ‘killed’ when passed in the current line.This uses the kill edge code. The end-of-page code signals the end ofthe page to the decoder.

Note that 7-bit and 13-bit runlengths are specifically chosen to support800 dpi A4/Letter pages. Longer runlengths could be supported withoutsignificant impact on compression performance. For example, ifsupporting 1600 dpi compression, the runlengths should be at least 8-bitand 14-bit respectively. A general-purpose choice might be 8-bit and16-bit, thus supporting up to 40″ wide 1600 dpi pages.

The full set of codes is defined in Table 11. Note that there is noend-of-line code. The decoder uses the page width to detect the end ofthe line. The lengths of the codes are ordered by the relativeprobabilities of the codes' occurrence.

TABLE 11 EDRL codewords code encoding suffix description Δ0 1 — don'tmove corresponding edge Δ+1 010 — move corresponding edge +1 Δ−1 011 —move corresponding edge −1 Δ+2 00010 — move corresponding edge +2 Δ−200011 — move corresponding edge −2 kill edge 0010 — kill correspondingedge create near edge 0011  7-bit RL create edge from short runlength(RL) create far edge 00001 13-bit RL create edge from long runlength(RL) end-of-page (EOP) 000001 — end-of-page marker

FIG. 17 shows a simple encoding example. Note that the common situationof an all-white line following another all-white line is encoded using asingle bit (Δ0), and an all-black line following another all-black lineis encoded using two bits (Δ0, Δ0).

Note that the foregoing describes the compression format, not thecompression algorithm per se. A variety of equivalent encodings can beproduced for the same image, some more compact than others. For example,a pure runlength encoding conforms to the compression format. The goalof the compression algorithm is to discover a good, if not the best,encoding for a given image.

The following is a simple algorithm for producing the EDRL encoding of aline with reference to its predecessor.

#define SHORT_RUN PRECISION 7 // precision of short run #defineLONG_RUN_PRECISION13 // precision of long run EDRL_CompressLine ( ByteprevLine[ ], // previous (reference) // bi-level line Byte currLine[ ],// current (coding) bi-level line int lineLen, // line length BITSTREAMs // output (compressed) bitstream ) int prevEdge = 0 // current edgeoffset in // previous line int currEdge = 0 // current edge offset in //current line int codedEdge = currEdge // most recent coded (output) edgeint prevColor = 0 // current color in prev line // (0 = white) intcurrColor = 0 // current color in current line int prevRun // currentrun in previous line int currRun // current run in current line boolbUpdatePrevEdge = true // force first edge update bool bUpdateCurrEdge =true // force first edge update while (codedEdge < lineLen) // possiblyupdate current edge in previous line if (bUpdatePrevEdge) if (prevEdge <lineLen) prevRun = GetRun(prevLine, prevEdge, lineLen, prevColor) elseprevRun = 0 prevEdge += prevRun prevColor = !prevColor bUpdatePrevEdge =false // possibly update current edge in current line if(bUpdateCurrEdge) if (currEdge < lineLen) currRun = GetRun(currLine,currEdge, lineLen, currColor) else currRun = 0 currEdge += currRuncurrColor = !currColor bUpdateCurrEdge = false // output delta wheneverpossible, i.e. when // edge senses match, and delta is small enough if(prevColor == currColor) delta = currEdge − prevEdge if (abs(delta) <=MAX_DELTA) PutCode(s, EDGE_DELTA0 + delta) codedEdge = currEdgebUpdatePrevEdge = true bUpdateCurrEdge = true continue // kill unmatchededge in previous line if (prevEdge <= currEdge) PutCode (s, KILL_EDGE)bUpdatePrevEdge = true // create unmatched edge in current line if(currEdge <= prevEdge) PutCode(s, CREATE_EDGE) if (currRun < 128)PutCode(s, CREATE_NEAR_EDGE) PutBits (currRun, SHORT_RUN_PRECISION) elsePutCode(s, CREATE_FAR_EDGE) PutBits (currRun, LONG_RUN_PRECISION)codedEdge = currEdge bUpdateCurrEdge = true

Note that the algorithm is blind to actual edge continuity betweenlines, and may in fact match the “wrong” edges between two lines.Happily the compression format has nothing to say about this, since itdecodes correctly, and it is difficult for a “wrong” match to have adetrimental effect on the compression ratio.

For completeness the corresponding decompression algorithm is givenbelow. It forms the core of the EDRL Expander unit in the printercontroller chip (Section 7).

EDRL_DecompressLine ( BITSTREAM s, // input (compressed) bitstream ByteprevLine[ ], // previous (reference) bi-level line Byte currLine[ ], //current (coding) bi-level line int lineLen // line length ) int prevEdge= 0 // current edge offset in // previous line int currEdge = 0 //current edge offset in current line int prevColor = 0 // current colorin previous line // (0 = white) int currColor = 0 // current color incurrent line while (currEdge < lineLen) code = GetCode(s) switch (code)case EDGE_DELTA_MINUS2: case EDGE_DELTA_MINUS1: case EDGE_DELTA_0: caseEDGE_DELTA_PLUS1: case EDGE_DELTA_PLUS2: // create edge from delta intdelta = code − EDGE_DELTA_0 int run = prevEdge + delta − currEdgeFillBitRun (currLine, currEdge, currColor, run) currEdge += runcurrColor = !currColor prevEdge += GetRun(prevLine, prevEdge, lineLen,prevColor) prevColor = !prevColor case KILL_EDGE: // discard unusedreference edge prevEdge += GetRun(prevLine, prevEdge, lineLen,prevColor) prevColor = !prevColor case CREATE_NEAR_EDGE: caseCREATE_FAR_EDGE: // create edge explicitly int run if (code ==CREATE_NEAR_EDGE) run = GetBits(s, SHORT_RUN_PRECISION) else run =GetBits(s, LONG_RUN_PRECISION) FillBitRun (currLine, currEdge,currColor, run) currColor = !currColor currEdge += run6.2.3.3 EDRL Compression Performance

Table 12 shows the compression performance of Group 4 and EDRL on theCCITT test documents used to select the Group 4 algorithm. Each documentrepresents a single page scanned at 400 dpi. Group 4's superiorperformance is due to its entropy-coded runlengths, tuned to 400 dpifeatures.

TABLE 12 Group 4 and EDRL compression performance on standard CCITTTdocuments at 400 dpi Group 4 EDRL CCITT document number compressionratio compression ratio 1 29.1 21.6 2 49.9 41.3 3 17.9 14.1 4 7.3 5.5 515.8 12.4 6 31.0 25.5 7 7.4 5.3 8 26.7 23.4

Magazine text is typically typeset in a typeface with serifs (such asTimes) at a point size of 10. At this size an A4/Letter page holds up to14,000 characters, though a typical magazine page holds only about 7,000characters. Text is seldom typeset at a point size smaller than 5. At800 dpi, text cannot be meaningfully rendered at a point size lower than2 using a standard typeface. Table 13 illustrates the legibility ofvarious point sizes.

TABLE 13 Text at different point sizes point size sample text (in Times)2

3

4

5

6

7

8

9

10

Table 14 shows Group 4 and EDRL compression performance on pages of textof varying point sizes, rendered at 800 dpi. Note that EDRL achieves therequired compression ratio of 2.5 for an entire page of text typeset ata point size of 3. The distribution of characters on the test pages isbased on English-language statistics.

TABLE 14 Group 4 and EDRL compression performance on text at 800 dpicharacters/A4 Group 4 EDRL compression point size page compression ratioratio 2 340,000 2.3 1.7 3 170,000 3.2 2.5 4 86,000 4.7 3.8 5 59,000 5.54.9 6 41,000 6.5 6.1 7 28,000 7.7 7.4 8 21,000 9.1 9.0 9 17,000 10.210.4 10 14,000 10.9 11.3 11 12,000 11.5 12.4 12 8,900 13.5 14.8 13 8,20013.5 15.0 14 7,000 14.6 16.6 15 5,800 16.1 18.5 20 3,400 19.8 23.9

For a point size of 9 or greater, EDRL slightly outperforms Group 4,simply because Group 4's runlength codes are tuned to 400 dpi.

These compression results bear out the observation that entropy-encodedrunlengths contribute much less to compression than 2D encoding, unlessthe data is poorly correlated vertically, such as in the case of verysmall characters.

6.2.4 Contone Layer Compression

6.2.4.1 JPEG Compression

The JPEG compression algorithm lossily compresses a contone image at aspecified quality level. It introduces imperceptible image degradationat compression ratios below 5:1, and negligible image degradation atcompression ratios below 10:1.

JPEG typically first transforms the image into a color space whichseparates luminance and chrominance into separate color channels. Thisallows the chrominance channels to be subsampled without appreciableloss because of the human visual system's relatively greater sensitivityto luminance than chrominance. After this first step, each color channelis compressed separately.

The image is divided into 8×8 pixel blocks. Each block is thentransformed into the frequency domain via a discrete cosine transform(DCT). This transformation has the effect of concentrating image energyin relatively lower-frequency coefficients, which allowshigher-frequency coefficients to be more crudely quantized. Thisquantization is the principal source of compression in JPEG. Furthercompression is achieved by ordering coefficients by frequency tomaximize the likelihood of adjacent zero coefficients, and thenrunlength-encoding runs of zeroes. Finally, the runlengths and non-zerofrequency coefficients are entropy coded. Decompression is the inverseprocess of compression.

6.2.4.2 CMYK Contone JPEG Compression Format

The CMYK contone layer is compressed to an interleaved color JPEGbytestream. The interleaving is required for space-efficientdecompression in the printer, but may restrict the decoder to two setsof Huffman tables rather than four (i.e. one per color channel). Ifluminance and chrominance are separated, then the luminance channels canshare one set of tables, and the chrominance channels the other set.

If luminance/chrominance separation is deemed necessary, either for thepurposes of table sharing or for chrominance subsampling, then CMY isconverted to YCrCb and Cr and Cb are duly subsampled. K is treated as aluminance channel and is not subsampled.

The JPEG bytestream is complete and self-contained. It contains all datarequired for decompression, including quantization and Huffman tables.

7 Printer Controller

7.1 Printer Controller Architecture

A printer controller 178 (FIG. 18) consists of the CePrint centralprocessor (CCP) chip 180, a 64 MBit RDRAM 182, and a master QA chip 184.

The CCP 180 contains a general-purpose processor 181 and a set ofpurpose-specific functional units controlled by the processor via aprocessor bus 186. Only three functional units are non-standard—an EDRLexpander 188, a halftoner/compositor 190, and a printhead interface 192which controls the Memjet printhead 143.

Software running on the processor 181 coordinates the various functionalunits to receive, expand and print pages. This is described in the nextsection. The various functional units of the CCP 180 are described insubsequent sections.

7.2 Page Expansion and Printing

Page expansion and printing proceeds as follows. A page description isreceived from the host via a host interface 194 and is stored in mainmemory 182. 6 MB of main memory 182 is dedicated to page storage. Thiscan hold two pages each not exceeding 3 MB, or one page up to 6 MB. Ifthe host generates pages not exceeding 3 MB, then the printer operatesin streaming mode—i.e. it prints one page while receiving the next. Ifthe host generates pages exceeding 3 MB, then the printer operates insingle-page mode—i.e. it receives each page and prints it beforereceiving the next. If the host generates pages exceeding 6 MB then theyare rejected by the printer. In practice the printer driver preventsthis from happening.

A page consists of two parts—the bi-level black layer, and the contonelayer. These are compressed in distinct formats—the bi-level black layerin EDRL format, the contone layer in JPEG format. The first stage ofpage expansion consists of decompressing the two layers in parallel. Thebi-level layer is decompressed by the EDRL expander unit 188, thecontone layer by a JPEG decoder 196.

The second stage of page expansion consists of halftoning the contoneCMYK data to bi-level CMYK, and then compositing the bi-level blacklayer over the bi-level CMYK layer. The halftoning and compositing iscarried out by the halftoner/compositor unit 190.

Finally, the composited bi-level CMYK image is printed via the printheadinterface unit 192, which controls the Memjet printhead 143.

Because the Memjet printhead 143 prints at high speed, the paper 144must move past the printhead 143 at a constant velocity. If the paper144 is stopped because data cannot be fed to the printhead 143 fastenough, then visible printing irregularities will occur. It is thereforeimportant to transfer bi-level CMYK data to the printhead interface 192at the required rate.

A fully-expanded 1600 dpi bi-level CMYK page has an image size of 119MB. Because it is impractical to store an expanded page in printermemory, each page is expanded in real time during printing. Thus thevarious stages of page expansion and printing are pipelined. The pageexpansion and printing data flow is described in Table 15. The aggregatetraffic to/from main memory via an interface 198 of 182 MB/s is wellwithin the capabilities of current technologies such as Rambus.

TABLE 15 Page expansion and printing data flow input output input outputprocess input window output window rate rate Receive — — JPEG 1 — 1.5MB/s contone stream — 3.5 Mp/s receive — — EDRL 1 — 1.5 MB/s bi-levelstream — 31 Mp/s decompress JPEG — 32-bit 8 1.5 MB/s 13 MB/s contonestream CMYK 3.5 Mp/s 3.5 Mp/s decompress EDRL — 1-bit K 1 1.5 MB/s 15MB/s bi-level stream 31 Mp/s^(a) 124 Mp/s halftone 32-bit 1 —^(b) — 13MB/s — CMYK 3.5 Mp/s^(c) — composite 1-bit 1 4-bit 1 15 MB/s 60 MB/s KCMYK 124 Mp/s 124 Mp/s print 4-bit 24,1^(d) — — 60 MB/s — CMYK 124 Mp/s— 91 MB/s 91 MB/S 182 MB/S ^(a)800 dpi → 1600 dpi (2 × 2 expansion)^(b)halftone combines with composite, so there is no external data flowbetween them ^(c)267 dpi → 1600 dpi (6 × 6 expansion) ^(d)Needs a windowof 24 lines, but only advances 1 line.

Each stage communicates with the next via a shared FIFO in main memory182. Each FIFO is organized into lines, and the minimum size (in lines)of each FIFO is designed to accommodate the output window (in lines) ofthe producer and the input window (in lines) of the consumer.Inter-stage main memory buffers are described in Table 16. The aggregatebuffer space usage of 6.3 MB leaves 1.7 MB free for program code andscratch memory (out of the 8 MB available).

TABLE 16 Page expansion and printing main memory buffers organizationnumber buffer buffer and line size of lines size compressed page bufferbyte stream —  6 MB (one or two pages) — contone CMYK buffer 32-bitinterleaved 8 × 2 = 16 142 KB CMYK (267 ppi × 8.5″ × 32 = 8.9 KB)bi-level K buffer 1-bit K 1 × 2 = 2   3 KB (1600 dpi = 8.5″ × 1 = 1.7 B)bi-level CMYK buffer 4-bit planar odd/even 24 + 1 = 25 166 KB CMYK (1600dpi × 8.5″ × 4 = 6.6 KB)  6.3 MBThe overall data flow, including FIFOs, is illustrated in FIG. 19.

Contone page decompression is carried out by the JPEG decoder 196.Bi-level page decompression is carried out by the EDRL expander 188.Halftoning and compositing is carried out by the halftoner/compositorunit 190. These functional units are described in the followingsections.

7.2.1 DMA Approach

Each functional unit contains one or more on-chip input and/or outputFIFOs. Each FIFO is allocated a separate channel in a multi-channel DMAcontroller 200. The DMA controller 200 handles single-address ratherthan double-address transfers, and so provides a separaterequest/acknowledge interface for each channel.

Each functional unit stalls gracefully whenever an input FIFO isexhausted or an output FIFO is filled.

The processor 181 programs each DMA transfer. The DMA controller 200generates the address for each word of the transfer on request from thefunctional unit connected to the channel. The functional unit latchesthe word onto or off the data bus 186 when its request is acknowledgedby the DMA controller 200. The DMA controller 200 interrupts theprocessor 181 when the transfer is complete, thus allowing the processor181 to program another transfer on the same channel in a timely fashion.

In general the processor 181 will program another transfer on a channelas soon as the corresponding main memory FIFO is available (i.e.non-empty for a read, non-full for a write).

The granularity of channel servicing implemented in the DMA controller200 depends somewhat on the latency of main memory 182.

7.2.2 EDRL Expander

The EDRL expander unit (EEU) 188 is shown in greater detail in FIG. 20.The unit 188 decompresses an EDRL-compressed bi-level image.

The input to the EEU 188 is an EDRL bitstream. The output from the EEUis a set of bi-level image lines, scaled horizontally from theresolution of the expanded bi-level image by an integer scale factor to1600 dpi.

Once started, the EEU 188 proceeds until it detects an end-of-page codein the EDRL bitstream, or until it is explicitly stopped via its controlregister.

The EEU 188 relies on an explicit page width to decode the bitstream.This must be written to a page width register 202 prior to starting theEEU 188.

The scaling of the expanded bi-level image relies on an explicit scalefactor. This must be written to a scale factor register 204 prior tostarting the EEU 188.

TABLE 17 EDRL expander control and configuration registers registerwidth description start 1 Start the EEU. stop 1 Stop the EEU. page width13 Page width used during decoding to detect end-of-line. scale factor 4Scale factor used during scaling of expanded image.

The EDRL compression format is described in Section 6.2.3.2. Itrepresents a bi-level image in terms of its edges. Each edge in eachline is coded relative to an edge in the previous line, or relative tothe previous edge in the same line. No matter how it is coded, each edgeis ultimately decoded to its distance from the previous edge in the sameline. This distance, or runlength, is then decoded to the string of onebits or zero bits which represent the corresponding part of the image.The decompression algorithm is defined in Section 6.2.3.2.

The EEU 188 consists of a bitstream decoder 206, a state machine 208,edge calculation logic 210, two runlength decoders 212, and a runlength(re)encoder 214. The bitstream decoder 206 decodes an entropy-codedcodeword from the bitstream and passes it to the state machine 208. Thestate machine 208 returns the size of the codeword to the bitstreamdecoder 206, which allows the decoder 206 to advance to the nextcodeword. In the case of a create edge code, the state machine 208 usesthe bitstream decoder 206 to extract the corresponding runlength fromthe bitstream. The state machine 208 controls the edge calculation logic210 and runlength decoding/encoding as defined in Table 19.

The edge calculation logic 210 is quite simple. The current edge offsetin the previous (reference) and current (coding) lines are maintained ina reference edge register 216 and edge register 218 respectively. Therunlength associated with a create edge code is output directly to therunlength decoder 212.1, and is added to the current edge. A delta codeis translated into a runlength by adding the associated delta to thereference edge and subtracting the current edge. The generated runlengthis output to the runlength decoder 212.1, and is added to the currentedge. The next runlength is extracted from the runlength encoder 214 andadded to the reference edge. A kill edge code simply causes the currentreference edge to be skipped. Again the next runlength is extracted fromthe runlength encoder 214 and added to the reference edge.

Each time the edge calculation logic 210 generates a runlengthrepresenting an edge, it is passed to the runlength decoder 212.1. Whilethe runlength decoder 212.1 decodes the run it generates a stall signalto the state machine 208. Since the runlength decoder 212 is slower thanthe edge calculation logic 210, there is not much point in decouplingit. The expanded line accumulates in a line buffer 220 large enough tohold an 8.5″ 800 dpi line (850 bytes).

The previously expanded line is also buffered in a buffer 222. It actsas a reference for the decoding of the current line. The previous lineis re-encoded as runlengths on demand. This is less expensive thanbuffering the decoded runlengths of the previous line, since the worstcase is one 13-bit runlength for each pixel (20 KB at 1600 dpi). Whilethe runlength encoder 214 encodes the run it generates a stall signal tothe state machine 208. The runlength encoder 214 uses the page width todetect end-of-line. The (current) line buffer 220 and the previous linebuffer 222 are concatenated and managed as a single FIFO to simplify therunlength encoder 214.

The second runlength decoder 212.2 decodes the output runlength to aline buffer 224 large enough to hold an 8.5″ 1600 dpi line (1700 bytes).The runlength passed to this output runlength decoder 212.2 ismultiplied by the scale factor from the register 204, so this decoder212.2 produces 1600 dpi lines. The line is output scale factor timesthrough the output pixel FIFO. This achieves the required verticalscaling by simple line replication. The EEU 188 could be designed withedge smoothing integrated into its image scaling. A simple smoothingscheme based on template-matching can be very effective. This wouldrequire a multi-line buffer between the low-resolution runlength decoderand the smooth scaling unit, but would eliminate the high-resolutionrunlength decoder.

7.2.2.1 EDRL Stream Decoder

The EDRL stream decoder 206 (FIG. 21) decodes entropy-coded EDRLcodewords in the input bitstream. It uses a two-byte input buffer 226viewed through a 16-bit barrel shifter 228 whose left (most significant)edge is always aligned to a codeword boundary in the bitstream. Adecoder 230 connected to the barrel shifter 228 decodes a codewordaccording to Table 18, and supplies the state machine 208 with thecorresponding code.

TABLE 18 EDRL stream codeword decoding table input codeword bit outputcode pattern^(a) output code bit pattern 1xxx xxxx Δ0 1 0000 0000 010xxxxx Δ+1 0 1000 0000 011x xxxx Δ−1 0 0100 0000 0010 xxxx kill edge 00010 0000 0011 xxxx create near edge 0 0001 0000 0001 0xxx Δ+2 0 00001000 0001 1xxx Δ−2 0 0000 0100 0000 1xxx create far edge 0 0000 00100000 01xx end-of-page (EOP) 0 0000 0001 ^(a)x = don't care

The state machine 208 in turn outputs the length of the code. This isadded, modulo-8, by an accumulator 232 to the current codeword bitoffset to yield the next codeword bit offset. The bit offset in turncontrols the barrel shifter 228. If the codeword bit offset wraps, thenthe carry bit controls the latching of the next byte from the inputFIFO. At this time byte 2 is latched to byte 1, and the FIFO output islatched to byte 2. It takes two cycles of length 8 to fill the inputbuffer. This is handled by starting states in the state machine 208.

7.2.2.2 EDRL Expander State Machine

The EDRL expander state machine 208 controls the edge calculation andrunlength expansion logic in response to codes supplied by the EDRLstream decoder 206. It supplies the EDRL stream decoder 206 with thelength of the current codeword and supplies the edge calculation logic210 with the delta value associated with the current delta code. Thestate machine 206 also responds to start and stop control signals from acontrol register 234 (FIG. 20), and the end-of-line (EOL) signal fromthe edge calculation logic 210.

The state machine 208 also controls the multi-cycle fetch of therunlength associated with a create edge code.

TABLE 19 EDRL expander state machine input input current next codesignal code state state len delta actions start — stopped starting 8 — —— — starting idle 8 — — stop — — stopped 0 — reset RL decoders and FIFOsEOL — — EOL 1 0 — reset RL encoder; reset RL decoders; reset ref. edgeand edge — — EOL 1 idle RL encoder → ref. RL; ref. edge + ref. RL → ref.edge — Δ0 idle idle 1 0 edge − ref. edge + delta → RL; edge + RL → edge;RL → RL decoder; RL encoder → ref. RL; ref. edge + ref. RL → ref. edge —Δ + 1 idle idle 2 +1 edge − ref. edge + delta → RL; edge + RL → edge; RL→ RL decoder; RL encoder → ref. RL; ref. edge + ref. RL → ref. edge — Δ− 1 idle idle 3 −1 edge − ref. edge + delta → RL; edge + RL → edge; RL →RL decoder; RL encoder → ref. RL; ref. edge + ref. RL → ref. edge — Δ +2 idle idle 4 +2 edge − ref. edge + delta → RL; edge + RL → edge; RL →RL decoder; RL encoder → ref. RL; ref. edge + ref. RL → ref. edge — Δ −2 idle idle 5 −2 edge − ref. edge + delta → RL; edge + RL → edge; RL →RL decoder; RL encoder → ref. RL; ref. edge + ref. RL → ref. edge — killidle idle 6 — RL encoder → edge ref. RL; ref. edge + ref. RL → ref. edge— create idle create 7 — reset create RL near RL lo 7 edge — create idlecreate 8 — — far edge RL hi 6 — EOP idle stopped 8 — — — — create create6 — latch RL hi 6 RL lo 7 create RL hi 6 — — create create 7 — latch RLlo 7 edge create RL lo 7 — — create idle 0 — create RL → RL; edge edge +RL → edge; RL → RL encoder7.2.2.3 Runlength Decoder

The runlength decoder 212 expands a runlength into a sequence of zerobits or one bits of the corresponding length in the output stream. Thefirst run in a line is assumed to be white (color 0). Each run isassumed to be of the opposite color to its predecessor. If the first runis actually black (color 1), then it must be preceded by a zero-lengthwhite run. The runlength decoder 212 keeps track of the current colorinternally.

The runlength decoder 212 appends a maximum of 8 bits to the outputstream every clock. Runlengths are typically not an integer multiple of8, and so runs other than the first in an image are typically notbyte-aligned. The runlength decoder 212 maintains, in a byte spaceregister 236 (FIG. 22), the number of bits available in the bytecurrently being built. This is initialized to 8 at the beginning ofdecoding, and on the output of every byte.

The decoder 212 starts outputting a run of bits as soon as the next runline 248 latches a non-zero value into a runlength register 238. Thedecoder 212 effectively stalls when the runlength register 238 goes tozero.

A number of bits of the current color are shifted into an output byteregister 240 each clock. The current color is maintained in a 1-bitcolor register 242. The number of bits actually output is limited by thenumber of bits left in the runlength, and by the number of spare bitsleft in the output byte. The number of bits output is subtracted fromthe runlength and the byte space. When the runlength goes to zero it hasbeen completely decoded, although the trailing bits of the run may stillbe in the output byte register 240, pending output. When the byte spacegoes to zero the output byte is full and is appended to the outputstream.

A 16-bit barrel shifter 244, the output byte register 240 and the colorregister 242 together implement an 8-bit shift register which can beshifted multiple bit positions every clock, with the color as the serialinput.

An external reset line 246 is used to reset the runlength decoder 212 atthe start of a line. An external next run line 248 is used to requestthe decoding of a new runlength. It is accompanied by a runlength on anexternal runlength line 250. The next run line 248 should not be set onthe same clock as the reset line 246. Because next run inverts thecurrent color, the reset of the color sets it to one, not zero. Anexternal flush line 252 is used to flush the last byte of the run, ifincomplete. It can be used on a line-by-line basis to yield byte-alignedlines, or on an image basis to yield a byte-aligned image.

An external ready line 254 indicates whether the runlength decoder 212is ready to decode a runlength. It can be used to stall the externallogic.

7.2.2.4 Runlength Encoder

The runlength encoder 214 detects a run of zero or one bits in the inputstream. The first run in a line is assumed to be white (color 0). Eachrun is assumed to be of the opposite color to its predecessor. If thefirst run is actually black (color 1), then the runlength encoder 214generates a zero-length white run at the start of the line. Therunlength decoder keeps track of the current color internally.

The runlength encoder 214 reads a maximum of 8 bits from the inputstream every clock. It uses a two-byte input buffer 256 (FIG. 23) viewedthrough a 16-bit barrel shifter 258 whose left (most significant) edgeis always aligned to the current position in the bitstream. An encoder260 connected to the barrel shifter 258 encodes an 8-bit (partial)runlength according to Table 20. The 8-bit runlength encoder 260 usesthe current color to recognize runs of the appropriate color.

The 8-bit runlength generated by the 8-bit runlength encoder 260 isadded to the value in a runlength register 262. When the 8-bit runlengthencoder 260 recognizes the end of the current run it generates anend-of-run signal which is latched by a ready register 264. The outputof the ready register 264 indicates that the encoder 214 has completedencoding the current runlength, accumulated in the runlength register262. The output of the ready register 264 is also used to stall the8-bit runlength encoder 260. When stalled the 8-bit runlength encoder260 outputs a zero-length run and a zero end-of-run signal, effectivelystalling the entire runlength encoder 214.

TABLE 20 8-bit runlength encoder table Color input length end-of-run 00000 0000 8 0 0 0000 0001 7 1 0 0000 001x 6 1 0 0000 01xx 5 1 0 00001xxx 4 1 0 0001 xxxx 3 1 0 001x xxxx 2 1 0 01xx xxxx 1 1 0 1xxx xxxx 0 11 1111 1111 8 0 1 1111 1110 7 1 1 1111 110x 6 1 1 1111 10xx 5 1 1 11110xxx 4 1 1 1110 xxxx 3 1 1 110x xxxx 2 1 1 10xx xxxx 1 1 1 0xxx xxxx 0 1

The output of the 8-bit runlength encoder 260 is limited by theremaining page width. The actual 8-bit runlength is subtracted from theremaining page width, and is added to a modulo-8 bit positionaccumulator 266 used to control the barrel shifter 258 and clock thebyte stream input.

An external reset line 268 is used to reset the runlength encoder 214 atthe start of a line. It resets the current color and latches a pagewidth signal on line 270 into a page width register 272. An externalnext run line 274 is used to request another runlength from therunlength encoder 214. It inverts the current color, and resets therunlength register 262 and ready register 264. An external flush line276 is used to flush the last byte of the run, if incomplete. It can beused on a line-by-line basis to process byte-aligned lines, or on animage basis to process a byte-aligned image.

An external ready line 278 indicates that the runlength encoder 214 isready to encode a runlength, and that the current runlength is availableon a runlength line 280. It can be used to stall the external logic.

7.2.2.5 Timing

The EEU 188 has an output rate of 124 M 1-bit black pixels/s. The corelogic generates one runlength every clock. The runlength decoders 212and the runlength encoder 214 generate/consume up to 8 pixels (bits) perclock. One runlength decoder 212.1 and the runlength encoder 214 operateat quarter resolution (800 dpi). The other runlength decoder 212.2operates at full resolution (1600 dpi).

A worst-case bi-level image consisting of a full page of 3 point textconverts to approximately 6 M runlengths at 800 dpi (the renderingresolution). At 1600 dpi (the horizontal output resolution) this givesan average runlength of about 20. Consequently about 40% of 8-pixeloutput bytes span two runs and so require two clocks instead of one.Output lines are replicated vertically to achieve a vertical resolutionof 1600 dpi. When a line is being replicated rather than generated ithas a perfect efficiency of 8 pixels per clock, thus the overhead ishalved to 20%.

The full-resolution runlength decoder in the output stage of the EEU 188is the slowest component in the EEU 188. The minimum clock speed of theEEU 188 is therefore dictated by the output pixel rate of the EEU (124Mpixels/s), divided by the width of the runlength decoder (8), adjustedfor its worst-case overhead (20%). This gives a minimum speed of about22 MHz.

7.2.3 JPEG Decoder

The JPEG decoder 196 (FIG. 24) decompresses a JPEG-compressed CMYKcontone image.

The input to the JPEG decoder 196 is a JPEG bitstream. The output fromthe JPEG decoder 196 is a set of contone CMYK image lines.

When decompressing, the JPEG decoder 196 writes its output in the formof 8×8 pixel blocks. These are sometimes converted to full-width linesvia an page width×8 strip buffer closely coupled with the codec. Thiswould require a 67 KB buffer. We instead use 8 parallel pixel FIFOs 282with shared bus access and 8 corresponding DMA channels, as shown inFIG. 24.

7.2.3.1 Timing

The JPEG decoder 196 has an output rate of 3.5 M 32-bit CMYK pixels/s.The required clock speed of the decoder depends on the design of thedecoder.

7.2.4 Halftoner/Compositor

The halftoner/compositor unit (HCU) 190 (FIG. 25) combines the functionsof halftoning the contone CMYK layer to bi-level CMYK, and compositingthe black layer over the halftoned contone layer.

The input to the HCU 190 is an expanded 267 ppi CMYK contone layer, andan expanded 1600 dpi black layer. The output from the HCU 190 is a setof 1600 dpi bi-level CMYK image lines.

Once started, the HCU 190 proceeds until it detects an end-of-pagecondition, or until it is explicitly stopped via its control register284.

The HCU 190 generates a page of dots of a specified width and length.The width and length must be written to page width and page lengthregisters of the control registers 284 prior to starting the HCU 190.The page width corresponds to the width of the printhead. The pagelength corresponds to the length of the target page.

The HCU 190 generates target page data between specified left and rightmargins relative to the page width. The positions of the left and rightmargins must be written to left margin and right margin registers of thecontrol registers 284 prior to starting the HCU 190. The distance fromthe left margin to the right margin corresponds to the target pagewidth.

The HCU 190 consumes black and contone data according to specified blackand contone page widths. These page widths must be written to black pagewidth and contone page width registers of the control registers 284prior to starting the HCU 190. The HCU 190 clips black and contone datato the target page width. This allows the black and contone page widthsto exceed the target page width without requiring any specialend-of-line logic at the input FIFO level.

The relationships between the page width, the black and contone pagewidths, and the margins are illustrated in FIG. 26.

The HCU 190 scales contone data to printer resolution both horizontallyand vertically based on a specified scale factor. This scale factor mustbe written to a contone scale factor register of the control registers284 prior to starting the HCU 190.

TABLE 21 Halftoner/compositor control and configuration registersregister width description start 1 Start the HCU. stop 1 Stop the HCU.page width 14 Page width of printed page, in dots. This is the number ofdots which have to be generated for each line. left margin 14 Positionof left margin, in dots. right margin 14 Position of right margin, indots. page length 15 Page length of printed page, in dots. This is thenumber of lines which have to be generated for each page. black pagewidth 14 Page width of black layer, in dots. Used to detect the end of ablack line. contone page 14 Page width of contone layer, in dots. Usedto width detect the end of a contone line. contone 4 Scale factor usedto scale contone data to scale factor bi-level resolution.

The consumer of the data produced by the HCU 190 is the printheadinterface 192. The printhead interface 192 requires bi-level CMYK imagedata in planar format, i.e. with the color planes separated. Further, italso requires that even and odd pixels are separated. The output stageof the HCU 190 therefore uses 8 parallel pixel FIFOs 286, one each foreven cyan, odd cyan, even magenta, odd magenta, even yellow, odd yellow,even black, and odd black.

An input contone CMYK FIFO 288 is a full 9 KB line buffer. The line isused contone scale factor times to effect vertical up-scaling via linereplication. FIFO write address wrapping is disabled until the start ofthe last use of the line. An alternative is to read the line from mainmemory contone scale factor times, increasing memory traffic by 44 MB/s,but avoiding the need for the on-chip 9 KB line buffer.

7.2.4.1 Multi-Threshold Dither

A multi-threshold dither unit 290 is shown in FIG. 27 of the drawings. Ageneral 256-layer dither volume provides great flexibility in dithercell design, by decoupling different intensity levels. General dithervolumes can be large—a 64×64×256 dither volume, for example, has a sizeof 128 KB. They are also inefficient to access since each colorcomponent requires the retrieval of a different bit from the volume. Inpractice, there is no need to fully decouple each layer of the dithervolume. Each dot column of the volume can be implemented as a fixed setof thresholds rather than 256 separate bits. Using three 8-bitthresholds, for example, only consumes 24 bits. Now, n thresholds definen+1 intensity intervals, within which the corresponding dither celllocation is alternately not set or set. The contone pixel value beingdithered uniquely selects one of the n+1 intervals, and this determinesthe value of the corresponding output dot.

We dither the contone data using a triple-threshold 64×64×3×8-bit (12KB) dither volume. The three thresholds form a convenient 24-bit valuewhich can be retrieved from the dither cell ROM in one cycle. If dithercell registration is desired between color planes, then the sametriple-threshold value can be retrieved once and used to dither eachcolor component. If dither cell registration is not desired, then thedither cell can be split into four subcells and stored in fourseparately addressable ROMs from which four different triple-thresholdvalues can be retrieved in parallel in one cycle. Using the addressingscheme shown below, the four color planes share the same dither cell atvertical and/or horizontal offsets of 32 dots from each other.

Each triple-threshold unit 292 converts a triple-threshold value and anintensity value into an interval and thence a one or zero bit. Thetriple-thresholding rules are shown in Table 22. The corresponding logicis shown in FIG. 28.

TABLE 22 Triple-thresholding rules interval output V ≦ T₁ 0 T₁ < V ≦ T₂1 T₂ < V ≦ T₃ 0 T₃ < V 17.2.4.2 Composite

A composite unit 294 of the HCU 190 composites a black layer dot over ahalftoned CMYK layer dot. If the black layer opacity is one, then thehalftoned CMY is set to zero.

Given a 4-bit halftoned color C_(c)M_(c)Y_(c)K_(c) and a 1-bit blacklayer opacity K_(b), the composite logic is as defined in Table 23.

TABLE 23 Composite logic color channel condition C C_(c)

 

K_(b) M M_(c)

 

K_(b) Y Y_(c)

 

K_(b) K K_(c)

 K_(b)7.2.4.3 Clock Enable Generator

A clock enable generator 296 of the HCU 190 generates enable signals forclocking the contone CMYK pixel input, the black dot input, and the CMYKdot output.

As described earlier, the contone pixel input buffer is used as both aline buffer and a FIFO. Each line is read once and then used contonescale factor times. FIFO write address wrapping is disabled until thestart of the final replicated use of the line, at which time the clockenable generator 296 generates a contone line advance enable signalwhich enables wrapping.

The clock enable generator 296 also generates an even signal which isused to select the even or odd set of output dot FIFOs, and a marginsignal which is used to generate white dots when the current dotposition is in the left or right margin of the page.

The clock enable generator 296 uses a set of counters. The internallogic of the counters is defined in Table 24. The logic of the clockenable signals is defined in Table 25.

TABLE 24 Clock enable generator counter logic load decrement counterabbr. w. data condition condition dot D 14 page width RP^(a)

 EOL^(b) (D>0)

 clk line L 15 page length RP (L>0)

 EOL left margin LM 14 left margin RP

 EOL (LM>0)

 clk right margin RM 14 right RP

 EOL (RM>0)

 clk margin even/odd dot E  1 0 RP

 EOL clk black dot BD 14 black width RP

 EOL (LM=0)

 (BD>0)

 clk contone dot CD 14 contone RP

 EOL (LM=0)

 (CD>0)

 clk width contone CSP  4 contone RP

 EOL

(LM=0)

 clk sub-pixel scale factor (CSP=0) contone CSL  4 contone RP

 (CSL=0) EOL

 clk sub-line scale factor ^(a)RP (reset page) condition: externalsignal ^(b)EOL (end-of-line) condition: (D=0)

 (BD=0)

 (CD=0)

TABLE 25 Clock enable generator output signal logic output signalcondition output dot clock enable (D>0)

 

EOP black dot clock enable (LM=0)

 (BD>0)

 

EOP contone pixel clock enable (LM=0)

 (CD>0)

 (CSP=0)

 

EOP contone line advance enable (CSL=0)

 

EOP even E=0 margin (LM=0)

 (RM=0) ^(a)EOP (end-of-page) condition: L=07.2.4.4 Timing

The HCU 190 has an output rate of 124 M 4-bit CMYK pixels/s. Since itgenerates one pixel per clock, it must be clocked at least 124 MHz.

7.3 Printhead Interface

CePrint 10 uses an 8.5″ CMYK Memjet printhead 143, as described inSection 9. The printhead consists of 17 segments arranged in 2 segmentgroups. The first segment group contains 9 segments, and the secondgroup contains 8 segments. There are 13,600 nozzles of each color in theprinthead 143, making a total of 54,400 nozzles. The printhead interface192 is a standard Memjet printhead interface, as described in Section10, configured with the following operating parameters:

MaxColors=4

SegmentsPerXfer=9

SegmentGroups=2

Although the printhead interface 192 has a number of externalconnections, not all are used for an 8.5″ printhead, so not all areconnected to external pins on the CCP 180. Specifically, the value forSegmentGroups implies that there are only 2 SRClock pins and 2SenseSegSelect pins. All 36 ColorData pins, however, are required.

7.3.1 Timing

CePrint 10 prints an 8.3″×11.7″ page in 2 seconds. The printhead 143must therefore print 18,720 lines (11.7″×1600 dpi) in 2 seconds, whichyields a line time of about 107 μs. Within the printhead interface 192,a single Print Cycle and a single Load Cycle must both complete withinthis time. In addition, the paper 144 must advance by about 16 μm in thesame time.

In high-speed print mode the Memjet printhead 143 can print an entireline in 100 μs. Since all segments fire at the same time 544 nozzles arefired simultaneously with each firing pulse. This leaves 7 μs for othertasks between each line.

The 1600 SRClock pulses (800 each of SRClock1 and SRClock2) to theprinthead 143 (SRClock1 has 36 bits of valid data, and SRClock2 has 32bits of valid data) must also take place within the 107 μs line time.Restricting the timing to 100 μs, the length of an SRClock pulse cannotexceed 1100 μs/1600=62.5 ns. The printhead 143 must therefore be clockedat 16 MHz.

The printhead interface 192 has a nominal pixel rate of 124 M 4-bit CMYKpixels/s. However, because it is only active for 100 μs out of every 107μs, it must be clocked at least 140 MHz. This can be increased to 144MHz to make it an integer multiple of the printhead speed.

7.4 Processor and Memory

7.4.1 Processor

The processor 181 runs the control program which synchronizes the otherfunctional units during page reception, expansion and printing. It alsoruns the device drivers for the various external interfaces, andresponds to user actions through the user interface.

It must have low interrupt latency, to provide efficient DMA management,but otherwise does not need to be particularly high-performance.

7.4.2 DMA Controller

The DMA controller 200 supports single-address transfers on 29 channels(see Table 26). It generates vectored interrupts to the processor 181 ontransfer completion.

TABLE 26 DMA channel usage functional unit input channels outputchannels host interface — 1 inter-CCP interface 1 1 EDRL expander 1 1JPEG decoder 1 8 halftoner/compositor 2 8 speaker interface 1 printheadinterface 4 — 10  19  29 7.4.3 Program ROM

A program ROM 298 holds the CCP 180 control program which is loaded intomain memory 182 during system boot.

7.4.4 Rambus Interface

The Rambus interface 198 provides the high-speed interface to theexternal 8 MB (64 Mbit) Rambus DRAM (RDRAM) 182.

7.5 External Interfaces

7.5.1 Host Interface

The host interface 194 provides a connection to the host processor witha speed of at least 1.5 MB/s (or 3 MB/s for the double-sided version ofCePrint).

7.5.2 Speaker Interface

A speaker interface 300 (FIG. 29) contains a small FIFO 302 used forDMA-mediated transfers of sound clips from main memory 182, an 8-bitdigital-to-analog converter (DAC) 304 which converts each 8-bit samplevalue to a voltage, and an amplifier 306 which feeds an external speaker308 (FIG. 18). When the FIFO 302 is empty it outputs a zero value.

The speaker interface 300 is clocked at the frequency of the soundclips.

The processor 181 outputs a sound clip to the speaker 308 simply byprogramming the DMA channel of the speaker interface 300.

7.5.3 Parallel Interface

A parallel interface 309 provides I/O on a number of parallel externalsignal lines. It allows the processor 181 to sense or control thedevices listed in Table 27.

TABLE 27 Parallel Interface devices parallel interface devices powerbutton power LED out-of-paper LED out-of-ink LED media sensor paperpick-up roller position sensor paper tray drive position sensor paperpick-up motor paper tray ejector motor transfer roller stepper motor7.5.4 Serial Interface

A serial interface 310 provides two standard low-speed serial ports. Oneport is used to connect to the master QA chip 184. The other is used toconnect to a QA chip 312 in the ink cartridge. The processor-mediatedprotocol between the two is used to authenticate the ink cartridge. Theprocessor 181 can then retrieve ink characteristics from the QA chip312, as well as the remaining volume of each ink. The processor 181 usesthe ink characteristics to properly configure the Memjet printhead 143.It uses the remaining ink volumes, updated on a page-by-page basis withink consumption information accumulated by the printhead interface 192,to ensure that it never allows the printhead 143 to be damaged byrunning dry.

7.5.4.1 Ink Cartridge QA Chip

The QA chip 312 in the ink cartridge 32 contains information requiredfor maintaining the best possible print quality, and is implementedusing an authentication chip. The 256 bits of data in the authenticationchip are allocated as follows:

TABLE 28 Ink cartridge's 256 bits (16 entries of 16-bits) M[n] accesswidth description 0 RO^(a) 16 Basic header, flags etc. 1 RO 16 Serialnumber. 2 RO 16 Batch number. 3 RO 16 Reserved for future expansion.Must be 0. 4 RO 16 Cyan ink properties. 5 RO 16 Magenta ink properties.6 RO 16 Yellow ink properties. 7 RO 16 Black ink properties. 8-9 DO^(b)32 Cyan ink remaining, in nanolitres. 10-11 DO 32 Magenta ink remaining,in nanolitres. 12-13 DO 32 Yellow ink remaining, in nanolitres. 14-15 DO32 Black ink remaining, in nanolitres. ^(a)read only (RO) ^(b)decrementonly

Before each page is printed, the processor 181 must check the amount ofink remaining to ensure there is enough for an entire worst-case page.Once the page has been printed, the processor 181 multiplies the totalnumber of drops of each color (obtained from the printhead interface192) by the drop volume. The amount of printed ink is subtracted fromthe amount of ink remaining. The unit of measurement for ink remainingis nanolitres, so 32 bits can represent over 4 liters of ink. The amountof ink used for a page must be rounded up to the nearest nanolitre (i.e.approximately 1000 printed dots).

7.5.5 Inter-CCP Interface

An inter-CCP interface 314 provides a bi-directional high-speed serialcommunications link to a second CCP, and is used in multi-CCPconfigurations such as the double-sided version of the printer whichcontains two CCPs.

The link has a minimum speed of 30 MB/s, to support timely distributionof page data, and may be implemented using a technology such as IEEE1394 or Rambus.

7.5.6 JTAG Interface

A standard JTAG (Joint Test Action Group) interface (not shown) isincluded for testing purposes. Due to the complexity of the chip, avariety of testing techniques are required, including BIST (Built InSelf Test) and functional block isolation. An overhead of 10% in chiparea is assumed for overall chip testing circuitry.

8 Double-Sided Printing

The double-sided version of CePrint contains two complete print enginesor printing units 140—one for the front of the paper, one for the back.Each printing unit 140 consists of a printer controller 178, a printheadassembly 142 containing a Memjet printhead 143, and a transfer roller68. Both printing units 140 share the same ink supply. The back side, orlower, printing unit 140 acts as the master unit. It is responsible forglobal printer functions, such as communicating with the host, handlingthe ink cartridge 32, handling the user interface, and controlling thepaper transport. The front side, or upper, printing unit 140 acts as aslave unit. It obtains pages from the host processor via the masterunit, and is synchronized by the master unit during printing.

Both printer controllers 178 consist of a CePrint central processor(CCP) 180 and a local 8 MB RDRAM 182. The external interfaces of themaster unit are used in the same way as in the single-sided version ofCePrint, but only the memory interface 198 and the printhead interface192 of the slave unit are used. An external master/slave pin on the CCP180 selects the mode of operation.

This dual printer controller configuration is illustrated in FIG. 30.

8.1 Page Delivery and Distribution

The master CCP 180M (FIG. 30) presents a unified view of the printer 10to the host processor. It hides the presence of the slave CCP 180S.

Pages are transmitted from the host processor to the printer 10 in pageorder. The first page of a document is always a front side page, andfront side and back side pages are always interleaved. Thus odd-numberedpages are front side pages, and even-numbered pages are back side pages.To print in single-sided mode on either the front side or back side ofthe paper, the host must send appropriate blank pages to the printer 10.The printer 10 expects a page description for every page.

When the master CCP 180M receives a page command from the host processorrelating to an odd-numbered page it routes the command to the slave CCP180S via the inter-CCP serial link 314. To avoid imposing unduerestrictions on the host link and its protocol, each command is receivedin its entirety and stored in the master's local memory 182M beforebeing forwarded to the memory 182S of the slave. This introduces only asmall delay because the inter-CCP link 314 is fast. To ensure that themaster CCP 180M always has a page buffer available for a page destinedfor the slave, the master is deliberately made the back side CCP, sothat it receives the front side odd-numbered page before it receives thematching back side even-numbered page.

8.2 Synchronized Printing

Once the master CCP 180M and the slave CCP 180S have received theirpages, the master CCP 180M initiates actual printing. This consists ofstarting the page expansion and printing processes in the master CCP180M, and initiating the same processes in the slave CCP 180S via acommand sent over the inter-CCP serial link 314.

To achieve perfect registration between the front side and back sideprinted pages, the printhead interfaces 192 of both CCPs aresynchronized to a common line synchronization signal. Thesynchronization signal is generated by the master CCP 180M.

Once the printing pipelines in both CCPs are sufficiently primed, asindicated by the stall status of the line loader/format unit (LLFU) ofthe printhead interface (Section 10.4), the master CCP 180M starts theline synchronization generator unit (LSGU) of the printhead interface192 (Section 10.2). The master CCP 180M obtains the status of the slaveCCP 180S LLFU via a poll sent over the inter-CCP serial link 314.

After the printing of a page, or more frequently, the master 180Mobtains ink consumption information from the slave 180S over theinter-CCP link 314. It uses this to update the remaining ink volume inthe ink cartridge 32, as described in Section 7.5.4.1.

The master and slave CCPs 180M, 180S also exchange error events andhost-initiated printer reset commands over the inter-CCP link 314.

9 Memjet Printhead

The Memjet printhead 143 is a drop-on-demand 1600 dpi inkjet printerthat produces bi-level dots in up to 4 colors to produce a printed pageof a particular width. Since the printhead prints dots at 1600 dpi, eachdot is approximately 22.5 mm in diameter, and spaced 15.875 mm apart.Because the printing is bi-level, the input image should be dithered orerror-diffused for best results.

Typically a Memjet printhead for a particular application is page-width.This enables the printhead 143 to be stationary and allows the paper 144to move past the printhead 143. FIG. 31 illustrates a typicalconfiguration.

The Memjet printhead 143 is composed of a number of identical ½ inchMemjet segments. The segment is therefore the basic building block forconstructing the printhead 143.

9.1 The Structure of a Memjet Segment

This section examines the structure of a single segment, the basicbuilding block for constructing the Memjet printhead 143.

9.1.1 Grouping of Nozzles within a Segment

The nozzles within a single segment are grouped for reasons of physicalstability as well as minimization of power consumption during printing.In terms of physical stability, a total of 10 nozzles share the same inkreservoir. In terms of power consumption, groupings are made to enable alow-speed and a high-speed printing mode. Memjet segments support twoprinting speeds to allow speed/power consumption trade-offs to be madein different product configurations.

In the low-speed printing mode, 4 nozzles of each color are fired fromthe segment at a time. The exact number of nozzles fired depends on howmany colors are present in the printhead. In a four color (e.g. CMYK)printing environment this equates to 16 nozzles firing simultaneously.In a three color (e.g. CMY) printing environment this equates to 12nozzles firing simultaneously. To fire all the nozzles in a segment, 200different sets of nozzles must be fired.

In the high-speed printing mode, 8 nozzles of each color are fired fromthe segment at a time. The exact number of nozzles fired depends on howmany colors are present in the printhead. In a four color (e.g. CMYK)printing environment this equates to 32 nozzles firing simultaneously.In a three color (e.g. CMY) printing environment this equates to 24nozzles firing simultaneously. To fire all the nozzles in a segment, 100different sets of nozzles must be fired.

The power consumption in the low-speed mode is half that of thehigh-speed mode. Note, however, that the energy consumed to print a pageis the same in both cases.

9.1.1.1 Ten Nozzles Make a Pod

A single pod consists of 10 nozzles sharing a common ink reservoir. 5nozzles are in one row, and 5 are in another. Each nozzle produces dots22.5 mm in diameter spaced on a 15.875 mm grid to print at 1600 dpi.FIG. 32 shows the arrangement of a single pod, with the nozzles numberedaccording to the order in which they must be fired.

Although the nozzles are fired in this order, the relationship ofnozzles and physical placement of dots on the printed page is different.The nozzles from one row represent the even dots from one line on thepage, and the nozzles on the other row represent the odd dots from theadjacent line on the page. FIG. 33 shows the same pod with the nozzlesnumbered according to the order in which they must be loaded.

The nozzles within a pod are therefore logically separated by the widthof 1 dot. The exact distance between the nozzles will depend on theproperties of the Memjet firing mechanism. The printhead 143 is designedwith staggered nozzles designed to match the flow of paper.

9.1.1.2 One Pod of Each Color Makes a Chromapod

One pod of each color are grouped together into a chromapod. The numberof pods in a chromapod will depend on the particular application. In amonochrome printing system (such as one that prints only black), thereis only a single color and hence a single pod. Photo printingapplication printheads require 3 colors (cyan, magenta, yellow), soMemjet segments used for these applications will have 3 pods perchromapod (one pod of each color). The expected maximum number of podsin a chromapod is 4, as used in a CMYK (cyan, magenta, yellow, black)printing system (such as a desktop printer). This maximum of 4 colors isnot imposed by any physical constraints—it is merely an expected maximumfrom the expected applications (of course, as the number of colorsincreases the cost of the segment increases and the number of theselarger segments that can be produced from a single silicon waferdecreases).

A chromapod represents different color components of the same horizontalset of 10 dots on different lines. The exact distance between differentcolor pods depends on the Memjet operating parameters, and may vary fromone Memjet design to another. The distance is considered to be aconstant number of dot-widths, and must therefore be taken into accountwhen printing: the dots printed by the cyan nozzles will be fordifferent lines than those printed by the magenta, yellow or blacknozzles. The printing algorithm must allow for a variable distance up toabout 8 dot-widths between colors. FIG. 34 illustrates a singlechromapod for a CMYK printing application.

9.1.1.3 Five Chromapods Make a Podgroup

Five chromapods are organized into a single podgroup. A podgrouptherefore contains 50 nozzles for each color. The arrangement is shownin FIG. 35, with chromapods numbered 0-4 and using a CMYK chromapod asthe example. Note that the distance between adjacent chromapods isexaggerated for clarity.

9.1.1.4 Two Podgroups Make a Phasegroup

Two podgroups are organized into a single phasegroup. The phasegroup isso named because groups of nozzles within a phasegroup are firedsimultaneously during a given firing phase (this is explained in moredetail below). The formation of a phasegroup from 2 podgroups isentirely for the purposes of low-speed and high-speed printing via 2PodgroupEnable lines.

During low-speed printing, only one of the two PodgroupEnable lines isset in a given firing pulse, so only one podgroup of the two firesnozzles. During high-speed printing, both PodgroupEnable lines are set,so both podgroups fire nozzles. Consequently a low-speed print takestwice as long as a high-speed print, since the high-speed print firestwice as many nozzles at once.

FIG. 36 illustrates the composition of a phasegroup. The distancebetween adjacent podgroups is exaggerated for clarity.

9.1.1.5 Two Phasegroups Make a Firegroup

Two phasegroups (PhasegroupA and PhasegroupB) are organized into asingle firegroup, with 4 firegroups in each segment. Firegroups are sonamed because they all fire the same nozzles simultaneously. Two enablelines, AEnable and BEnable, allow the firing of PhasegroupA nozzles andPhasegroupB nozzles independently as different firing phases. Thearrangement is shown in FIG. 37. The distance between adjacent groupingsis exaggerated for clarity.

9.1.1.6 Nozzle Grouping Summary

Table 29 is a summary of the nozzle groupings in a segment assuming aCMYK chromapod.

TABLE 29 Nozzle Groupings for a single segment Name of ReplicationGrouping Composition Ratio Nozzle Count Nozzle Base unit 1:1 1 PodNozzles per pod 10:1  10 Chromapod Pods per chromapod C:1 10C PodgroupChromapods per podgroup 5:1 50C Phasegroup Podgroups per phasegroup 2:1100C Firegroup Phasegroups per firegroup 2:1 200C Segment Firegroups persegment 4:1 800C

The value of C, the number of colors contained in the segment,determines the total number of nozzles.

With a 4 color segment, such as CMYK, the number of nozzles per segmentis 3,200.

With a 3 color segment, such as CMY, the number of nozzles per segmentis 2,400.

In a monochrome environment, the number of nozzles per segment is 800.

9.1.2 Load and Print Cycles

A single segment contains a total of 800C nozzles, where C is the numberof colors in the segment. A Print Cycle involves the firing of up to allof these nozzles, dependent on the information to be printed. A LoadCycle involves the loading up of the segment with the information to beprinted during the subsequent Print Cycle.

Each nozzle has an associated NozzleEnable bit that determines whetheror not the nozzle will fire during the Print Cycle. The NozzleEnablebits (one per nozzle) are loaded via a set of shift registers.

Logically there are C shift registers per segment (one per color), each800 deep. As bits are shifted into the shift register for a given colorthey are directed to the lower and upper nozzles on alternate pulses.Internally, each 800-deep shift register is comprised of two 400-deepshift registers: one for the upper nozzles, and one for the lowernozzles. Alternate bits are shifted into the alternate internalregisters. As far as the external interface is concerned however, thereis a single 800 deep shift register.

Once all the shift registers have been fully loaded (800 load pulses),all of the bits are transferred in parallel to the appropriateNozzleEnable bits. This equates to a single parallel transfer of 800Cbits. Once the transfer has taken place, the Print Cycle can begin. ThePrint Cycle and the Load Cycle can occur simultaneously as long as theparallel load of all NozzleEnable bits occurs at the end of the PrintCycle.

9.1.2.1 Load Cycle

The Load Cycle is concerned with loading the segment's shift registerswith the next Print Cycle's NozzleEnable bits.

Each segment has C inputs directly related to the C shift registers(where C is the number of colors in the segment). These inputs are namedColorNData, where N is 1 to C (for example, a 4 color segment would have4 inputs labeled Color1Data, Color2Data, Color3Data and Color4Data). Asingle pulse on the SRClock line transfers C bits into the appropriateshift registers. Alternate pulses transfer bits to the lower and uppernozzles respectively. A total of 800 pulses are required for thecomplete transfer of data. Once all 800C bits have been transferred, asingle pulse on the PTransfer line causes the parallel transfer of datafrom the shift registers to the appropriate NozzleEnable bits.

The parallel transfer via a pulse on PTransfer must take place after thePrint Cycle has finished. Otherwise the NozzleEnable bits for the linebeing printed will be incorrect.

It is important to note that the odd and even dot outputs, althoughprinted during the same Print Cycle, do not appear on the same physicaloutput line. The physical separation of odd and even nozzles within theprinthead, as well as separation between nozzles of different colorsensures that they will produce dots on different lines of the page. Thisrelative difference must be accounted for when loading the data into theprinthead 143. The actual difference in lines depends on thecharacteristics of the inkjet mechanism used in the printhead 143. Thedifferences can be defined by variables D₁ and D₂ where D₁ is thedistance between nozzles of different colors, and D₂ is the distancebetween nozzles of the same color. Table 30 shows the dots transferredto a C color segment on the first 4 pulses.

TABLE 30 Order of Dots Transferred to a Segment Pulse Dot Color1 LineColor2 Line Color3 Line ColorC Line 1 0 N N+D₁ ^(a) N+2D₁ N+(C−1)D₁ 2 1N+D₂ ^(b) N+D1+D₂ N+2D₁ +D₂ N+(C−1)D₁ +D₂ 3 2 N N+D₁ N+2D₁ N+(C−1)D₁ 4 3N+D₂ N+D₁ +D₂ N+2D₁ +D₂ N+(C−1)D₁ +D₂ ^(a)D₁ =number of lines betweenthe nozzles of one color and the next (likely =4-8) ^(b)D₂ =number oflines between two rows of nozzles of the same color (likely=1)

And so on for all 800 pulses.

Data can be clocked into a segment at a maximum rate of 20 MHz, whichwill load the entire 800C bits of data in 40 μs.

9.1.2.2 Print Cycle

A single Memjet printhead segment contains 800 nozzles. To fire them allat once would consume too much power and be problematic in terms of inkrefill and nozzle interference. This problem is made more apparent whenwe consider that a Memjet printhead is composed of multiple ½ inchsegments, each with 800 nozzles. Consequently two firing modes aredefined: a low-speed printing mode and a high-speed printing mode:

-   -   In the low-speed print mode, there are 200 phases, with each        phase firing 4C nozzles (C per firegroup, where C is the number        of colors).

In the high-speed print mode, there are 100 phases, with each phasefiring 8C nozzles, (2C per firegroup, where C is the number of colors).The nozzles to be fired in a given firing pulse are determined by

-   -   3 bits ChromapodSelect (select 1 of 5 chromapods from a        firegroup)    -   4 bits NozzleSelect (select 1 of 10 nozzles from a pod)    -   2 bits of PodgroupEnable lines (select 0, 1, or 2 podgroups to        fire)

When one of the PodgroupEnable lines is set, only the specifiedPodgroup's 4 nozzles will fire as determined by ChromapodSelect andNozzleSelect. When both of the PodgroupEnable lines are set, both of thepodgroups will fire their nozzles. For the low-speed mode, two firepulses are required, with PodgroupEnable=10 and 01 respectively. For thehigh-speed mode, only one fire pulse is required, withPodgroupEnable=11.

The duration of the firing pulse is given by the AEnable and BEnablelines, which fire the PhasegroupA and PhasegroupB nozzles from allfiregroups respectively. The typical duration of a firing pulse is1.3-1.8 ms. The duration of a pulse depends on the viscosity of the ink(dependent on temperature and ink characteristics) and the amount ofpower available to the printhead 143. See Section 9.1.3 for details onfeedback from the printhead 143 in order to compensate for temperaturechange.

The AEnable and BEnable are separate lines in order that the firingpulses can overlap. Thus the 200 phases of a low-speed Print Cycleconsist of 100 A phases and 100 B phases, effectively giving 100 sets ofPhase A and Phase B. Likewise, the 100 phases of a high-speed printcycle consist of 50 A phases and 50 B phases, effectively giving 50phases of phase A and phase B.

FIG. 38 shows the AEnable and BEnable lines during a typical PrintCycle. In a high-speed print there are 50 2 ms cycles, while in alow-speed print there are 100 2 μs cycles.

For the high-speed printing mode, the firing order is:

ChromapodSelect 0, NozzleSelect 0, PodgroupEnable 11 (Phases A and B)ChromapodSelect 1, NozzleSelect 0, PodgroupEnable 11 (Phases A and B)ChromapodSelect 2, NozzleSelect 0, PodgroupEnable 11 (Phases A and B)ChromapodSelect 3, NozzleSelect 0, PodgroupEnable 11 (Phases A and B)ChromapodSelect 4, NozzleSelect 0, PodgroupEnable 11 (Phases A and B)ChromapodSelect 0, NozzleSelect 1, PodgroupEnable 11 (Phases A and B) .. . ChromapodSelect 3, NozzleSelect 9, PodgroupEnable 11 (Phases A andB) ChromapodSelect 4, NozzleSelect 9, PodgroupEnable 11 (Phases A and B)

For the low-speed printing mode, the firing order is similar. For eachphase of the high speed mode where PodgroupEnable was 11, two phases ofPodgroupEnable=01 and 10 are substituted as follows:

ChromapodSelect 0, NozzleSelect 0, PodgroupEnable 01 (Phases A and B)ChromapodSelect 0, NozzleSelect 0, PodgroupEnable 10 (Phases A and B)ChromapodSelect 1, NozzleSelect 0, PodgroupEnable 01 (Phases A and B)ChromapodSelect 1, NozzleSelect 0, PodgroupEnable 10 (Phases A and B) .. . ChromapodSelect 3, NozzleSelect 9, PodgroupEnable 01 (Phases A andB) ChromapodSelect 3, NozzleSelect 9, PodgroupEnable 10 (Phases A and B)ChromapodSelect 4, NozzleSelect 9, PodgroupEnable 01 (Phases A and B)ChromapodSelect 4, NozzleSelect 9, PodgroupEnable 10 (Phases A and B)

When a nozzle fires, it takes approximately 100 μs to refill. The nozzlecannot be fired before this refill time has elapsed. This limits thefastest printing speed to 100 μs per line. In the high-speed print mode,the time to print a line is 100 μs, so the time between firing a nozzlefrom one line to the next matches the refill time. The low-speed printmode is slower than this, so is also acceptable.

The firing of a nozzle also causes acoustic perturbations for a limitedtime within the common ink reservoir of that nozzle's pod. Theperturbations can interfere with the firing of another nozzle within thesame pod. Consequently, the firing of nozzles within a pod should beoffset from each other as long as possible. We therefore fire fournozzles from a chromapod (one nozzle per color) and then move onto thenext chromapod within the podgroup.

-   -   In the low-speed printing mode the podgroups are fired        separately. Thus the 5 chromapods within both podgroups must all        fire before the first chromapod fires again, totaling 10×2 μs        cycles. Consequently each pod is fired once per 20 μs.    -   In the high-speed printing mode, the podgroups are fired        together. Thus the 5 chromapods within a single podgroups must        all fire before the first chromapod fires again, totaling 5×2 μs        cycles. Consequently each pod is fired once per 10 μs.

As the ink channel is 300 mm long and the velocity of sound in the inkis around 1500 m/s, the resonant frequency of the ink channel is 2.5MHz. Thus the low-speed mode allows 50 resonant cycles for the acousticpulse to dampen, and the high-speed mode allows 25 resonant cycles.Consequently any acoustic interference is minimal in both cases.

9.1.3 Feedback from a Segment

A segment produces several lines of feedback. The feedback lines areused to adjust the timing of the firing pulses. Since multiple segmentsare collected together into a printhead, it is effective to share thefeedback lines as a tri-state bus, with only one of the segments placingthe feedback information on the feedback lines.

A pulse on the segment's SenseSegSelect line ANDed with data onColor1Data selects if the particular segment will provide the feedback.The feedback sense lines will come from that segment until the nextSenseSegSelect pulse. The feedback sense lines are as follows:

-   -   Tsense informs the controller how hot the printhead is. This        allows the controller to adjust timing of firing pulses, since        temperature affects the viscosity of the ink.    -   Vsense informs the controller how much voltage is available to        the actuator. This allows the controller to compensate for a        flat battery or high voltage source by adjusting the pulse        width.    -   Rsense informs the controller of the resistivity (Ohms per        square) of the actuator heater. This allows the controller to        adjust the pulse widths to maintain a constant energy        irrespective of the heater resistivity.    -   Wsense informs the controller of the width of the critical part        of the heater, which may vary up to ±5% due to lithographic and        etching variations. This allows the controller to adjust the        pulse width appropriately.        9.1.4 Preheat Cycle

The printing process has a strong tendency to stay at the equilibriumtemperature. To ensure that the first section of a printed image, suchas a photograph, has a consistent dot size, the equilibrium temperaturemust be met before printing any dots. This is accomplished via a preheatcycle.

The Preheat cycle involves a single Load Cycle to all nozzles of asegment with 1s (i.e. setting all nozzles to fire), and a number ofshort firing pulses to each nozzle. The duration of the pulse must beinsufficient to fire the drops, but enough to heat up the ink.Altogether about 200 pulses for each nozzle are required, cyclingthrough in the same sequence as a standard Print Cycle.

Feedback during the Preheat mode is provided by Tsense, and continuesuntil equilibrium temperature is reached (about 30° C. above ambient).The duration of the Preheat mode is around 50 milliseconds, and dependson the ink composition.

Preheat is performed before each print job. This does not affectperformance as it is done while the data is being transferred to theprinter.

9.1.5 Cleaning Cycle

In order to reduce the chances of nozzles becoming clogged, a cleaningcycle can be undertaken before each print job. Each nozzle is fired anumber of times into an absorbent sponge.

The cleaning cycle involves a single Load Cycle to all nozzles of asegment with 1s (i.e. setting all nozzles to fire), and a number offiring pulses to each nozzle. The nozzles are cleaned via the samenozzle firing sequence as a standard Print Cycle. The number of timesthat each nozzle is fired depends upon the ink composition and the timethat the printer has been idle. As with preheat, the cleaning cycle hasno effect on printer performance.

9.1.6 Printhead Interface Summary

Each segment has the following connections to the bond pads:

TABLE 31 Segment Interface Connections Name Lines Description ChromapodSelect  3 Select which chromapod will fire (0-4) NozzleSelect  4 Selectwhich nozzle from the pod will fire (0-9). PodgroupEnable  2 Enable thepodgroups to fire (choice of: 01, 10, 11) AEnable  1 Firing pulse forpodgroup A BEnable  1 Firing pulse for podgroup B ColorNData  C Input toshift registers (1 bit for each of C colors in the segment) SRClock  1 Apulse on SRClock (ShiftRegisterClock) loads C bits from ColorData intothe C shift registers. PTransfer  1 Parallel transfer of data from theshift registers to the internal NozzleEnable bits (one per nozzle).SenseSegSelect  1 A pulse on SenseSegSelect ANDed with data onColor1Data selects the sense lines for this segment. Tsense  1Temperature sense Vsense  1 Voltage sense Rsense  1 Resistivity senseWsense  1 Width sense Logic GND  1 Logic ground Logic PWR  1 Logic powerV− 21 Actuator Ground V+ 21 Actuator Power TOTAL 62+C (if C is 4, Total=66)9.2 Making Memjet Printheads Out of Segments

A Memjet printhead is composed of a number of identical ½ inch printheadsegments. These ½ inch segments are manufactured together or placedtogether after manufacture to produce a printhead of the desired length.Each ½ inch segments prints 800 1600 dpi bi-level dots in up to 4 colorsover a different part of the page to produce the final image. Althougheach segment produces 800 dots of the final image, each dot isrepresented by a combination of colored inks.

A 4-inch printhead, for example, consists of 8 segments, typicallymanufactured as a monolithic printhead. In a typical 4-color printingapplication (cyan, magenta, yellow, black), each of the segments printsbi-level cyan, magenta, yellow and black dots over a different part ofthe page to produce the final image.

An 8-inch printhead can be constructed from two 4-inch printheads orfrom a single 8-inch printhead consisting of 16 segments. Regardless ofthe construction mechanism, the effective printhead is still 8 inches inlength.

A 2-inch printhead has a similar arrangement, but only uses 4 segments.Likewise, a full-bleed A4/Letter printer uses 17 segments for aneffective 8.5 inch printing area.

Since the total number of nozzles in a segment is 800C (see Table 29),the total number of nozzles in a given printhead with S segments is800CS. Thus segment N is responsible for printing dots 800N to 800N+799.

A number of considerations must be made when wiring up a printhead. Asthe width of the printhead increases, the number of segments increases,and the number of connections also increases. Each segment has its ownColorData connections (C of them), as well as SRClock and otherconnections for loading and printing.

9.2.1 Loading Considerations

When the number of segments S is small it is reasonable to load all thesegments simultaneously by using a common SRClock line and placing Cbits of data on each of the ColorData inputs for the segments. In a4-inch printer, S=8, and therefore the total number of bits to transferto the printhead in a single SRClock pulse is 32. However for an 8-inchprinter, S=16, and it is unlikely to be reasonable to have 64 data linesrunning from the print data generator to the printhead.

Instead, it is convenient to group a number of segments together forloading purposes. Each group of segments is small enough to be loadedsimultaneously, and share an SRClock. For example, an 8-inch printheadcan have 2 segment groups, each segment group containing 8 segments. 32ColorData lines can be shared for both groups, with 2 SRClock lines, oneper segment group.

When the number of segment groups is not easily divisible, it is stillconvenient to group the segments. One example is a 8.5 inch printer forproducing A4/Letter pages. There are 17 segments, and these can begrouped as two groups of 9 (9C bits of data going to each segment, withall 9C bits used in the first group, and only 8C bits used for thesecond group), or as 3 groups of 6 (again, C bits are unused in the lastgroup).

As the number of segment groups increases, the time taken to load theprinthead increases. When there is only one group, 800 load pulses arerequired (each pulse transfers C data bits). When there are G groups,800G load pulses are required. The bandwidth of the connection betweenthe data generator and the printhead must be able to cope and be withinthe allowable timing parameters for the particular application.

If G is the number of segment groups, and L is the largest number ofsegments in a group, the printhead requires LC ColorData lines and GSRClock lines. Regardless of G, only a single PTransfer line isrequired—it can be shared across all segments.

Since L segments in each segment group are loaded with a single SRClockpulse, any printing process must produce the data in the correctsequence for the printhead. As an example, when G=2 and L=4, the firstSRClock1 pulse will transfer the ColorData bits for the next PrintCycle's dot 0, 800, 1600, and 2400. The first SRClock2 pulse willtransfer the ColorData bits for the next Print Cycle's dot 3200, 4000,4800, and 5600. The second SRClock1 pulse will transfer the ColorDatabits for the next Print Cycle's dot 1, 801, 1601, and 2401. The secondSRClock2 pulse will transfer the ColorData bits for the next PrintCycle's dot 3201, 4001, 4801 and 5601.

After 800G SRClock pulses (800 to each of SRClock1 and SRClock2), theentire line has been loaded into the printhead, and the common PTransferpulse can be given.

It is important to note that the odd and even color outputs, althoughprinted during the same Print Cycle, do not appear on the same physicaloutput line. The physical separation of odd and even nozzles within theprinthead, as well as separation between nozzles of different colorsensures that they will produce dots on different lines of the page. Thisrelative difference must be accounted for when loading the data into theprinthead. The actual difference in lines depends on the characteristicsof the inkjet mechanism used in the printhead. The differences can bedefined by variables D₁ and D₂ where D₁ is the distance between nozzlesof different colors, and D₂ is the distance between nozzles of the samecolor. Considering only a single segment group, Table 32 shows the dotstransferred to segment n of a printhead during the first 4 pulses of theshared SRClock.

TABLE 32 Order of Dots Transferred to a Segment in a Printhead Pulse DotColor1 Line Color2 Line ColorC Line 1 800S^(a) N N+D₁ ^(b) N+(C−1)D₁ 2800S+1 N+D₂ ^(c) N+D₁ +D₂ N+(C−1)D₁ +D₂ 3 800S+2 N N+D₁ N+(C −0 1)D₁ 4800S+3 N+D₂ N+D₁ +D₂ N+(C −0 1)D₁ +D₂ ^(a)S = segment number ^(b)D₁ =number of lines between the nozzles of one color and the next(likely=4-8) ^(c)D₂ = number of lines between two rows of nozzles of thesame color (likely=1)

And so on for all 800 SRClock pulses to the particular segment group.

9.2.2 Printing Considerations

With regards to printing, we print 4C nozzles from each segment in thelow-speed printing mode, and 8C nozzles from each segment in the highspeed printing mode.

While it is certainly possible to wire up segments in any way, we onlyconsider the situation where all segments fire simultaneously. This isbecause the low-speed printing mode allows low-power printing for smallprintheads (e.g. 2-inch and 4-inch), and the controller chip designassumes there is sufficient power available for the large print sizes(such as 8-18 inches). It is a simple matter to alter the connections inthe printhead 143 to allow grouping of firing should a particularapplication require it.

When all segments are fired at the same time 4CS nozzles are fired inthe low-speed printing mode and 8CS nozzles are fired in the high-speedprinting mode. Since all segments print simultaneously, the printinglogic is the same as defined in Section 9.1.2.2.

The timing for the two printing modes is therefore:

-   -   200 μs to print a line at low speed (comprised of 100 2 μs        cycles)    -   100 μs to print a line at high speed (comprised of 50 2 μs        cycles)        9.2.3 Feedback Considerations

A segment produces several lines of feedback, as defined in Section9.1.3. The feedback lines are used to adjust the timing of the firingpulses. Since multiple segments are collected together into a printhead,it is effective to share the feedback lines as a tri-state bus, withonly one of the segments placing the feedback information on thefeedback lines at a time.

Since the selection of which segment will place the feedback informationon the shared Tsense, Vsense, Rsense, and Wsense lines uses the Color1Data line, the groupings of segments for loading data can be used forselecting the segment for feedback. Just as there are G SRClock lines (asingle line is shared between segments of the same segment group), thereare G SenseSegSelect lines shared in the same way. When the correctSenseSegSelect line is pulsed, the segment of that group whose Color1Data bit is set will start to place data on the shared feedback lines.The segment previously active in terms of feedback must also be disabledby having a 0 on its Color1 Data bit, and this segment may be in adifferent segment group. Therefore when there is more than one segmentgroup. changing the feedback segment requires two steps: disabling theold segment, and enabling the new segment.

9.2.4 Printhead Connection Summary

This section assumes that the printhead 143 has been constructed from anumber of segments as described in the previous sections. It assumesthat for data loading purposes, the segments have been grouped into Gsegment groups, with L segments in the largest segment group. It assumesthere are C colors in the printhead. It assumes that the firingmechanism for the printhead 143 is that all segments firesimultaneously, and only one segment at a time places feedbackinformation on a common tri-state bus. Assuming all these things, Table33 lists the external connections that are available from a printhead:

TABLE 33 Printhead Connections Name #Pins Description ChromapodSelect 3Select which chromapod will fire (0-4) NozzleSelect 4 Select whichnozzle from the pod will fire (0-9) PodgroupEnable 2 Enable thepodgroups to fire (choice of: 01, 10, 11) AEnable 1 Firing pulse forphasegroup A BEnable 1 Firing pulse for phasegroup B ColorData CL Inputsto C shift registers of segments 0 to L−1 SRClock G A pulse onSRClock[N] (ShiftRegisterClock N) loads the current values fromColorData lines into the L segments in segment group N. PTransfer 1Parallel transfer of data from the shift registers to the internalNozzleEnable bits (one per nozzle). SenseSegSelect G A pulse onSenseSegSelect N ANDed with data on Color1Data[n] selects the senselines for segment n in segment group N. Tsense 1 Temperature senseVsense 1 Voltage sense Rsense 1 Resistivity sense Wsense 1 Width senseLogic GND 1 Logic ground Logic PWR 1 Logic power V− Bus Actuator Groundbars V+ Actuator Power TOTAL 18+2G+CL

10 Memjet Printhead Interface

The printhead interface (PHI) 192 is the means by which the processor181 loads the Memjet printhead 143 with the dots to be printed, andcontrols the actual dot printing process. The PHI 192 contains:

-   -   a LineSyncGen unit (LSGU), which provides synchronization        signals for multiple chips (allows side-by-side printing and        front/back printing) as well as stepper motors.    -   a Memjet interface (MJI), which transfers data to the Memjet        printhead, and controls the nozzle firing sequences during a        print.    -   a line loader/format unit (LLFU) which loads the dots for a        given print line into local buffer storage and formats them into        the order required for the Memjet printhead.

The units within the PHI 192 are controlled by a number of registersthat are programmed by the processor 181. In addition, the processor 181is responsible for setting up the appropriate parameters in the DMAcontroller 200 for the transfers from memory to the LLFU. This includesloading white (all 0's) into appropriate colors during the start and endof a page so that the page has clean edges.

The PHI 192 is capable of dealing with a variety of printhead lengthsand formats. In terms of broad operating customizations, the PHI 192 isparameterized as follows:

TABLE 34 Basic Printing Parameters Name Description Range MaxColors Noof Colors in printhead 1-4 SegmentsPerXfer No of segments written to pertransfer. Is 1-9 equal to the number of segments in the largest segmentgroup SegmentGroups No of segment groups in printhead 1-4

The internal structure of the PHI allows for a maximum of 4 colors, 9segments per transfer, and 4 transfers. Transferring 4 colors to 9segments is 36 bits per transfer, and 4 transfers to 9 segments equatesto a maximum printed line length of 18 inches. The total number of dotsper line printed by an 18-inch 4 color printhead is 115,200 (18×1600×4).

Other example settings are shown in Table 35:

TABLE 35 Example Settings for Basic Printing Parameters Printer LengthPrinter Type MaxColors SegmentsPerXfer SegmentGroups Bits Comments4-inch CMY Photo 3 8 1 24 8-inch CMYK A4/Letter 4 8 2 32 8.5 inch CMYKA4/Letter full bleed 4 9 2 36 Last xfer not fully used 12 inch CMYK A4long/A3 short 4 8 3 32 16 inch CMYK 4 8 4 32 17 inch CMYK A3 long fullbleed 4 9 4 36 Last xfer not fully used 18 inch CMYK 4 9 4 3610.1 Block Diagram of Printhead Interface

The internal structure of the Printhead Interface 192 is shown in FIG.39.

In the PHI 192 there are two LSGUs 316, 318. The first LSGU 316 producesLineSync0, which is used to control the Memjet Interface 320 in allsynchronized chips. The second LSGU 318 produces LineSync1 which is usedto pulse the paper drive stepper motor.

The Master/Slave pin on the chip allows multiple chips to be connectedtogether for side-by-side printing, front/back printing etc. via aMaster/Slave relationship. When the Master/Slave pin is attached to VDD,the chip is considered to be the Master, and LineSync pulses generatedby the two Line SyncGen units 316, 318 are enabled onto the twotri-state LineSync common lines (LineSync0 and LineSync1, shared by allthe chips). When the Master/Slave pin is attached to GND, the chip isconsidered to be the Slave, and LineSync pulses generated by the twoLineSyncGen units 316, 318 are not enabled onto the common LineSynclines. In this way, the Master chip's LineSync pulses are used by allPHIs 192 on all the connected chips.

The following sections detail the LineSyncGen Unit 316, 318, the LineLoader/Format Unit 322 and Memjet Interface 320 respectively.

10.2 LineSyncGen Unit

The LineSyncGen units (LSGU) 316, 318 are responsible for generating thesynchronization pulses required for printing a page. Each LSGU 316, 318produces an external LineSync signal to enable line synchronization. Thegenerator inside the LGSU 316, 318 generates a LineSync pulse when toldto ‘go’, and then every so many cycles until told to stop. The LineSyncpulse defines the start of the next line.

The exact number of cycles between LineSync pulses is determined by theCyclesBetweenPulses register, one per generator. It must be at leastlong enough to allow one line to print (100 μs or 200 ms depending onwhether the speed is low or high) and another line to load, but can belonger as desired (for example, to accommodate special requirements ofpaper transport circuitry). If the CyclesBetweenPulses register is setto a number less than a line print time, the page will not printproperly since each LineSync pulse will arrive before the particularline has finished printing.

The following interface registers are contained in each LSGU 316, 318:

TABLE 36 LineSyncGen Unit Registers Register Name DescriptionCyclesBetweenPulses The number of cycles to wait between generating oneLineSync pulse and the next. Go Controls whether the LSGU is currentlygenerating LineSync pulses or not. A write of 1 to this registergenerates a LineSync pulse, transfers CyclesBetweenPulses toCyclesRemaining, and starts the countdown. When CyclesRemaining hits 0,another LineSync pulse is generated, CyclesBetweenPulses is transferredto CyclesRemaining and the countdown is started again. A write of 0 tothis register stops the countdown and no more LineSync pulses aregenerated. CyclesRemaining A status register containing the number ofcycles remaining until the next LineSync pulse is generated.

The LineSync pulse is not used directly from the LGSU 316, 318. TheLineSync pulse is enabled onto a tri-state LineSync line only if theMaster/Slave pin is set to Master. Consequently the LineSync pulse isonly used in the form as generated by the Master chip (pulses generatedby Slave chips are ignored).

10.3 Memjet Interface

The Memjet interface (MJI) 320 transfers data to the Memjet printhead143, and controls the nozzle firing sequences during a print.

The MJI 320 is simply a State Machine (see FIG. 40) which follows theprinthead loading and firing order described in Section 9.2.1, Section9.2.2, and includes the functionality of the Preheat Cycle and CleaningCycle as described in Section 9.1.4 and Section 9.1.5. Both high-speedand low-speed printing modes are available, although the MJI 320 alwaysfires a given nozzle from all segments in a printhead simultaneously(there is no separate firing of nozzles from one segment and thenothers). Dot counts for each color are also kept by the MJI 320.

The MJI loads data into the printhead from a choice of 2 data sources:

-   -   All 1s. This means that all nozzles will fire during a        subsequent Print cycle, and is the standard mechanism for        loading the printhead for a preheat or cleaning cycle.    -   From the 36-bit input held in the Transfer register of the LLFU        322. This is the standard means of printing an image. The 36-bit        value from the LLFU 322 is directly sent to the printhead and a        1-bit ‘Advance’ control pulse is sent to the LLFU 322.

The MJI 320 knows how many lines it has to print for the page. When theMJI 320 is told to ‘go’, it waits for a LineSync pulse before it startsthe first line. Once it has finished loading/printing a line, it waitsuntil the next LineSync pulse before starting the next line. The MJI 320stops once the specified number of lines has been loaded/printed, andignores any further LineSync pulses.

The MJI 320 is therefore directly connected to the LLFU 322, LineSync0(shared between all synchronized chips), and the external Memjetprinthead 143.

The MJI 320 accepts 36 bits of data from the LLFU 322. Of these 36 bits,only the bits corresponding to the number of segments and number ofcolors will be valid. For example, if there are only 2 colors and 9segments, bits 0-1 will be valid for segment 0, bits 2-3 will beinvalid, bits 4-5 will be valid for segment 1, bits 6-7 will be invalidetc. The state machine does not care which bits are valid and which bitsare not valid—it merely passes the bits out to the printhead 143. Thedata lines and control signals coming out of the MJI 320 can be wiredappropriately to the pinouts of the chip, using as few pins as requiredby the application range of the chip (see Section 10.3.1 for moreinformation).

10.3.1 Connections to Printhead

The MJI 320 has a number of connections to the printhead 143, includinga maximum of 4 colors, clocked in to a maximum of 9 segments pertransfer to a maximum of 4 segment groups. The lines coming from the MJI320 can be directly connected to pins on the chip, although not alllines will always be pins. For example, if the chip is specificallydesigned for only connecting to 8 inch CMYK printers, only 32 bits ofdata need to be transferred each transfer pulse. Consequently 32 pins ofdata out (8 pins per color), and not 36 pins are required. In the sameway, only 2 SRClock pulses are required, so only 2 pins instead of 4pins are required to cater for the different SRClocks. And so on.

If the chip must be completely generic, then all connections from theMJI 320 must be connected to pins on the chip (and thence to the Memjetprinthead 143).

Table 37 lists the maximum connections from the MJI 320, many of whichare always connected to pins on the chip. Where the number of pins isvariable, a footnote explains what the number of pins depends upon. Thesense of input and output is with respect to the MJI 320. The namescorrespond to the pin connections on the printhead 143.

TABLE 37 Memjet Interface Connections Name #Pins I/O DescriptionChromapod Select 3 O Select which chromapod will fire (0-4) NozzleSelect4 O Select which nozzle from the pod will fire (0-9) PodgroupEnable 2 OEnable the podgroups to fire (choice of: 01, 10, 11) AEnable 1 O Firingpulse for podgroup A. In the current design all segments firesimultaneously, although multiple AEnable lines could be added fordividing the firing sequence over multiple segment groups for reasons ofpower and speed. BEnable 1 O Firing pulse for podgroup B. In the currentdesign all segments fire simultaneously, although multiple BEnable linescould be added for dividing the firing sequence over multiple segmentgroups for reasons of power and speed. Color1Data[0-8] 9^(a) O Output toColor1Data shift register of segments 0-8 Color2Data[0-8] 9^(b) O Outputto Color2Data shift register of segments 0-8 Color3Data[0-8] 9^(c) OOutput to Color3Data shift register of segments 0-8 Color4Data[0-8]9^(d) O Output to Color4Data shift register of segments 0-8 SRClock[1-4]4e O A pulse on SRClock[N] (ShiftRegisterClock) loads the current valuesfrom Color1Data[0-8], Color2Data[0-8], Color3Data[0-8] andColor4Data[0-8] into the segment group N on the printhead. PTransfer 1 OParallel transfer of data from the shift registers to the printhead'sinternal NozzleEnable bits (one per nozzle). SenseSegSelect[1-4] 4^(f) OA pulse on SenseSegSelect[N] ANDed with data on Color1Data[n] enablesthe sense lines for segment n in segment group N of the printhead.Tsense 1 I Temperature sense Vsense 1 I Voltage sense Rsense 1 IResistivity sense Wsense 1 I Width sense TOTAL 52 ^(a)Although 9 linesare available from the MJI, the number of pins coming from the chip willonly reflect the actual number of segments in a segment group. The pinsfor Color1Data are mandatory, since each printhead must print in atleast 1 color. ^(b)These lines are only translated into pins if the chipis to control a printhead with at least 2 colors. Although 9 lines areavailable from the MJI, the number of pins coming from the chip forColor2Data will only reflect the actual number of segments in a segmentgroup. ^(c)These lines are only translated into pins if the chip is tocontrol a printhead with at least 3 colors. Although 9 lines areavailable from the MJI, the number of pins coming from the chip forColor3Data will only reflect the actual number of segments in a segmentgroup. ^(d)These lines are only translated into pins if the chip is tocontrol a printhead with 4 colors. Although 9 lines are available fromthe MJI, the number of pins coming from the chip for Color4Data willonly reflect the actual number of segments in a segment group.^(e)Although 4 lines are available from the MJI, the number of pinscoming from the chip will only reflect the actual number of segmentgroups. A minimum of 1 pin is required since there is at least 1 segmentgroup (the entire printhead). ^(f)Although 4 lines are available fromthe MJI, the number of pins coming from the chip will only reflect theactual number of segment groups. A minimum of 1 pin is required sincethere is at least 1 segment group (the entire printhead).10.3.2 Firing Pulse Duration

The duration of firing pulses on the AEnable and BEnable lines depend onthe viscosity of the ink (which is dependant on temperature and inkcharacteristics) and the amount of power available to the printhead 143.The typical pulse duration range is 1.3 to 1.8 μs. The MJI 320 thereforecontains a programmable pulse duration table 324 (FIG. 41), indexed byfeedback from the printhead 143. The table 324 of pulse durations allowsthe use of a lower cost power supply, and aids in maintaining moreaccurate drop ejection.

The Pulse Duration table 324 has 256 entries, and is indexed by thecurrent Vsense and Tsense settings on lines 326 and 328, respectively.The upper 4-bits of address come from Vsense, and the lower 4-bits ofaddress come from Tsense. Each entry is 8 bits, and represents a fixedpoint value in the range of 0-4 ms. The process of generating theAEnable and BEnable lines is shown in FIG. 41.

The 256-byte table 324 is written by the processor 181 before printingthe first page. The table 324 may be updated in between pages ifdesired. Each 8-bit pulse duration entry in the table 324 combines:

-   -   User brightness settings (from the page description)    -   Viscosity curve of ink (from the QA Chip)    -   Rsense    -   Wsense    -   Vsense    -   Tsense        10.3.3 Dot Counts

The MJI 320 maintains a count of the number of dots of each color firedfrom the printhead 143. The dot count for each color is a 32-bit value,individually cleared under processor control. At 32-bits length, eachdot count can hold a maximum coverage dot count of 17 8-inch×12-inchpages, although in typical usage, the dot count will be read and clearedafter each page or half-page.

The dot counts are used by the processor 181 to update the QA chip 312(see Section 7.5.4) in order to predict when the ink cartridge 32 runsout of ink. The processor 181 knows the volume of ink in the cartridge32 for each of the colors from the QA chip 312. Counting the number ofdrops eliminates the need for ink sensors, and prevents the ink channelsfrom running dry. An updated drop count is written to the QA chip 312after each page. A new page will not be printed unless there is enoughink left, and allows the user to change the ink without getting a dudhalf-printed page which must be reprinted.

The layout of the dot counter for Color1 is shown in FIG. 42. Theremaining 3 dot counters (Color1DotCount, Color2DotCount, andColor3DotCount) are identical in structure.

10.3.4 Registers

The processor 181 communicates with the MJI 320 via a register set. Theregisters allow the processor 181 to parameterize a print as well asreceive feedback about print progress.

The following registers are contained in the MJI 320:

TABLE 38 Memjet Interface Registers Register Name Description PrintParameters SegmentsPerXfer The number of segments to write to eachtransfer. This also equals the number of cycles to wait between eachtransfer (before generating the next Advance pulse). Each transfer hasMaxColors × SegmentsPerXfer valid bits. SegmentGroups The number ofsegment groups in the printhead. This equals the number of times thatSegmentsPerXfer cycles must elapse before a single dot has been writtento each segment of the printhead. The MJI does this 800 times tocompletely transfer all the data for the line to the printhead.PrintSpeed Whether to print at low or high speed (determines the valueon the PodgroupEnable lines during the print). NumLines The number ofLoad/Print cycles to perform. Monitoring the Print (read only from pointof view of processor) Status The Memjet Interface's Status RegisterLinesRemaining The number of lines remaining to be printed. Only validwhile Go = 1. Starting value is NumLines and counts down to 0.TransfersRemaining The number of sets of SegmentGroups transfersremaining before the Printhead is considered loaded for the currentline. Starts at 800 and counts down to 0. Only valid while Go = 1.SegGroupsRemaining The number of segment groups remaining in the currentset of transfers of 1 dot to each segment. Starts at SegmentGroups andcounts down to 0. Only valid while Go = 1. SenseSegment The 9-bit valueto place on the Color1Data lines during a subsequent feedbackSenseSegSelect pulse. Only 1 of the 9 bits should be set, correspondingto one of the (maximum) 9 segments. See SenseSelect for how to determinewhich of the segment groups to sense. SetAllNozzles If non-zero, the36-bit value written to the printhead during the LoadDots process is all1s, so that all nozzles will be fired during the subsequent PrintDotsprocess. This is used during the preheat and cleaning cycles. If 0, the36-bit value written to the printhead comes from the LLFU. This is thecase during the actual printing of regular images. Actions Reset A writeto this register resets the MJI, stops any loading or printingprocesses, and loads all registers with 0. SenseSelect A write to thisregister with any value clears the Feedback Valid bit of the Statusregister, and the remaining action depends on the values in theLoadingDots and PrintingDots status bits. If either of the status bitsare set, the Feedback bit is cleared and nothing more is done. If bothstatus bits are clear, a pulse is given simultaneously on all 4SenseSegSelect lines with all ColorData bits 0. This stops any existingfeedback. Depending on the two low-order bits written to SenseSelectregister, a pulse is given on SenseSegSelect1, SenseSegSelect2,SenseSegSelect3, or SenseSegSelect4 line, with the Color1Data bits setaccording to the SenseSegment register. Once the various sense lineshave been tested, the values are placed in the Tsense, Vsense, Rsense,and Wsense registers, and the Feedback bit of the Status register isset. Go A write of 1 to this bit starts the LoadDots/ PrintDots cycles,which commences with a wait for the first LineSync pulse. A total ofNumLines lines are printed, each line being loaded/printed after thereceipt of a LineSync pulse. The loading of each line consists ofSegmentGroups 36-bit transfers. As each line is printed, LinesRemainingdecrements, and TransfersRemaining is reloaded with SegmentGroups again.The status register contains print status information. Upon completionof NumLines, the loading/printing process stops, the Go bit is cleared,and any further LineSync pulses are ignored. During the final printcycle, nothing is loaded into the printhead. A write of 0 to this bitstops the print process, but does not clear any other registers.ClearCounts A write to this register clears the Color1DotCount,Color2DotCount, Color3DotCount, and Color4DotCount registers if bits 0,1, 2, or 3 respectively are set. Consequently a write of 0 has noeffect. Feedback Tsense Read only feedback of Tsense from the lastSenseSegSelect pulse sent to segment SenseSegment. Is only valid if theFeedback Valid bit of the Status register is set. Vsense Read onlyfeedback of Vsense from the last SenseSegSelect pulse sent to segmentSenseSegment. Is only valid if the FeedbackValid bit of the Statusregister is set. Rsense Read only feedback of Rsense from the lastSenseSegSelect pulse sent to segment SenseSegment. Is only valid if theFeedback Valid bit of the Status register is set. Wsense Read onlyfeedback of Wsense from the last SenseSegSelect pulse sent to segmentSenseSegment. Is only valid if the Feedback Valid bit of the Statusregister is set. Color1DotCount Read only 32-bit count of color1 dotssent to the printhead. Color2DotCount Read only 32-bit count of color2dots sent to the printhead. Color3DotCount Read only 32-bit count ofcolor3 dots sent to the printhead Color4DotCount Read only 32-bit countof color4 dots sent to the printhead

The MJI's Status Register is a 16-bit register with bit interpretationsas follows:

TABLE 39 MJI Status Register Name Bits Description LoadingDots 1 If set,the MJI is currently loading dots, with the number of dots remaining tobe transferred in TransfersRemaining. If clear, the MJI is not currentlyloading dots PrintingDots 1 If set, the MJI is currently printing dots.If clear, the MJI is not currently printing dots. PrintingA 1 This bitis set while there is a pulse on the AEnable line PrintingB 1 This bitis set while there is a pulse on the BEnable line Feedback Valid 1 Thisbit is set while the feedback values Tsense, Vsense, Rsense, and Wsenseare valid. Reserved 3 — PrintingChromapod 4 This holds the currentchromapod being fired while the PrintingDots status bit is set.PrintingNozzles 4 This holds the current nozzle being fired while thePrintingDots status bit is set.

The following pseudocode illustrates the logic required to load aprinthead for a single line. Note that loading commences only after theLineSync pulse arrives. This is to ensure the data for the line has beenprepared by the LLFU 322 and is valid for the first transfer to theprinthead 143.

Wait for LineSync For TransfersRemaining = 800 to 0 For I = 0 toSegmentGroups If (SetAllNozzles) Set all ColorData lines to be 1 ElsePlace 36 bit input on 36 ColorData lines EndIf Pulse SRClock[I] WaitSegmentsPerXfer cycles Send ADVANCE signal EndFor EndFor10.3.5 Preheat and Cleaning Cycles

The Cleaning and Preheat cycles are simply accomplished by settingappropriate registers in the MJI 320:

-   -   SetAllNozzles=1    -   Set the PulseDuration register to either a low duration (in the        case of the preheat mode) or to an appropriate drop ejection        duration for cleaning mode.    -   Set NumLines to be the number of times the nozzles should be        fired    -   Set the Go bit and then wait for the Go bit to be cleared when        the print cycles have completed.

The LSGU 316, 318 must also be programmed to send LineSync pulses at thecorrect frequency.

10.4 Line Loader/Format Unit

The line loader/format unit (LLFU) 322 loads the dots for a given printline into local buffer storage and formats them into the order requiredfor the Memjet printhead 143. It is responsible for supplying thepre-calculated nozzleEnable bits to the Memjet interface 320 for theeventual printing of the page.

The printing uses a double buffering scheme for preparing and accessingthe dot-bit information. While one line is being loaded into the firstbuffer, the pre-loaded line in the second buffer is being read in Memjetdot order. Once the entire line has been transferred from the secondbuffer to the printhead 143 via the Memjet interface 320, the readingand writing processes swap buffers. The first buffer is now read and thesecond buffer is loaded up with the new line of data. This is repeatedthroughout the printing process, as can be seen in the conceptualoverview of FIG. 43.

The size of each buffer is 14 KBytes to cater for the maximum linelength of 18 inches in 4 colors (18×1600×4 bits=115,200 bits=14,400bytes). The size for both Buffer 0 (330—FIG. 44) and Buffer 1 (332) is28.128 KBytes. While this design allows for a maximum print length of 18inches, it is trivial to reduce the buffer size to target a specificapplication.

Since one buffer 330, 332 is being read from while the other is beingwritten to, two sets of address lines must be used. The 32-bits DataIn334 from the common data bus 186 are loaded depending on theWriteEnables, which are generated by State Machine 336 in response tothe DMA Acknowledges.

A multiplexor 338 chooses between the two 4-bit outputs of Buffer 0 andBuffer 1, and sends the result to a 9-entry by 4-bit shift register 340.After a maximum of 9 read cycles (the number depends on the number ofsegments written to per transfer), and whenever an Advance pulse comesfrom the MJI 320, the current 36-bit value from the shift register 340is gated into a 36-bit Transfer register 342, where it can be used bythe MJI 320.

Note that not all the 36 bits are necessarily valid. The number of validbits of 36 depends on the number of colors in the printhead 143, thenumber of segments, and the breakup of segment groups (if more than onesegment group). For more information, see Section 9.2.

A single line in an L-inch C-color printhead consists of 1600L C-colordots. At 1 bit per colored dot, a single print-line consists of 1600LCbits. The LLFU 322 is capable of addressing a maximum line size of 18inches in 4 colors, which equates to 108,800 bits (14 KBytes) per line.These bits must be supplied to the MJI 320 in the correct order forbeing sent on to the printhead 143. See Section 9.2.1 for moreinformation concerning the Load Cycle dot loading order, but in summary,2LC bits are transferred to the printhead 143 in SegmentGroupstransfers, with a maximum of 36 bits per transfer. Each transfer to aparticular segment of the printhead 143 must load all colorssimultaneously.

10.4.1 Buffers

Each of the two buffers 330, 332 is broken into 4 sub-buffers, 1 percolor. The size of each sub-buffer is 3600 bytes, enough to hold18-inches of single color dots at 1600 dpi. The memory is accessed32-bits at a time, so there are 900 addresses for each buffer (requiring10 bits of address).

All the even dots are placed before the odd dots in each color's buffer,as shown in FIG. 45. If there is any unused space it is placed at theend of each color's buffer.

The amount of memory actually used is directly related to the printheadlength. If the printhead is 18 inches, there are 1800 bytes of even dotsfollowed by 1800 bytes of odd dots, with no unused space. If theprinthead is 12 inches, there are 1200 bytes of even dots followed by1200 odd dots, and 1200 bytes unused.

The number of sub-buffers gainfully used is directly related to thenumber of colors in the printhead. This number is typically 3 or 4,although it is quite feasible for this system to be used in a 1 or 2color system (with some small memory wastage). In a desktop printingenvironment, the number of colors would be 4: Color1=Cyan,Color2=Magenta, Color3=Yellow, Color4=Black.

The address decoding circuitry is such that in a given cycle, a single32-bit access can be made to all 4 sub-buffers—either a read from all 4or a write to one of the 4. Only one bit of the 32-bits read from eachcolor buffer is selected, for a total of 4 output bits. The process isshown in FIG. 46. 15 bits of address allow the reading of a particularbit by means of 10-bits of address being used to select 32 bits, and5-bits of address choose 1-bit from those 32. Since all color buffersshare this logic, a single 15-bit address gives a total of 4 bits out,one per color. Each buffer has its own WriteEnable line, to allow asingle 32-bit value to be written to a particular color buffer in agiven cycle. The 32-bits of DataIn are shared, since only one bufferwill actually clock the data in.

Note that regardless of the number of colors in the printhead, 4 bitsare produced in a given read cycle (one bit from each color's buffer).

10.4.2 Address Generation

10.4.2.1 Reading

Address Generation for reading is straightforward. Each cycle wegenerate a bit address which is used to fetch 4 bits representing 1-bitper color for a particular segment. By adding 400 to the current bitaddress, we advance to the next segment's equivalent dot. We add 400(not 800) since the odd and even dots are separated in the buffer. We dothis firstly SegmentGroups sets of SegmentsPerXfer times to retrieve thedata representing the even dots (the dot data is transferred to the MJI36 bits at a time) and another SegmentGroups sets of SegmentsPerXfertimes to load the odd dots. This entire process is repeated 400 times,incrementing the start address each time. Thus all dot values aretransferred in the order required by the printhead in400×2×SegmentGroups×SegmentsPerXfer cycles.

In addition, we generate the TransferWriteEnable control signal. Sincethe LLFU 322 starts before the MJI 320, we must transfer the first valuebefore the Advance pulse from the MJI 320. We must also generate thenext value in readiness for the first Advance pulse. The solution is totransfer the first value to the Transfer register after SegmentsPerXfercycles, and then to stall SegmentsPerXfer-cycles later, waiting for theAdvance pulse to start the next SegmentsPerXfer cycle group. Once thefirst Advance pulse arrives, the LLFU 322 is synchronized to the MJI320. However, the LineSync pulse to start the next line must arrive atthe MJI 320 at least 2SegmentsPerXfer cycles after the LLFU 322 so thatthe initial Transfer value is valid and the next 32-bit value is readyto be loaded into the Transfer register 342.

The read process is shown in the following pseudocode:

DoneFirst = FALSE For DotInSegment0 = 0 to 400 CurrAdr = DotInSegment0XfersRemaining = 2 × SegmentGroups DotCount = SegmentsPerXfer Do V1 =DotCount = 0 TransferWriteEnable = (V1 AND NOT DoneFirst) OR ADVANCEStall = V1 AND (NOT TransferWriteEnable) If (NOT Stall) Shift Register =Fetch 4-bits from CurrReadBuffer:CurrAdr CurrAdr = CurrAdr + 400 If (V1)DotCount = SegmentsPerXfer − 1 XfersRemaining = XfersRemaining − 1 ElseDotCount = DotCount − 1 EndIf EndIf Until (XfersRemaining=0) AND (NOTStall) EndFor

The final transfer may not be fully utilized. This occurs when thenumber of segments per transfer does not divide evenly into the actualnumber of segments in the printhead. An example of this is the 8.5 inchprinthead, which has 17 segments. Transferring 9 segments each timemeans that only 8 of the last 9 segments will be valid. Nonetheless, thetiming requires the entire 9th segment value to be generated (eventhough it is not used). The actual address is therefore a don't carestate since the data is not used.

Once the line has finished, the CurrReadBuffer value must be toggled bythe processor.

10.4.2.2 Writing

The write process is also straightforward. 4 DMA request lines areoutput to the DMA controller 200. As requests are satisfied by thereturn DMA Acknowledge lines, the appropriate 8-bit destination addressis selected (the lower 5 bits of the 15-bit output address are don'tcare values) and the acknowledge signal is passed to the correctbuffer's WriteEnable control line (the Current Write Buffer isCurrentReadBuffer). The 10-bit destination address is selected from the4 current addresses, one address per color. As DMA requests aresatisfied the appropriate destination address is incremented, and thecorresponding TransfersRemaining counter is decremented. The DMA requestline is only set when the number of transfers remaining for that coloris non-zero. The following pseudocode illustrates the Write process:

CurrentAdr[1-4] = 0 While (ColorXfersRemaining[1-4] are non-zero)DMARequest[1-4] = ColorXfersRemaining[1-4] NOT = 0 If DMAAknowledge[N]CurrWriteBuffer:CurrentAdr[N] = Fetch 32-bits from data busCurrentAdr[N] = CurrentAdr[N] + 1 ColorXfersRemaining[N] =ColorXfersRemaining[N] − 1 (floor 0) EndIf EndWhile10.4.3 Registers

The following interface registers are contained in the LLFU 322:

TABLE 40 Line Load/Format Unit Registers Register Name DescriptionSegmentsPerXfer The number of segments whose dots must be loaded beforeeach transfer. This has a maximum value of 9. SegmentGroups The numberof segment groups in the printhead. This has a maximum number of 4.CurrentReadBuffer The current buffer being read from. When Buffer0 isbeing read, Buffer1 is written to and vice versa. Should be toggled witheach AdvanceLine pulse from the MJI. Go Bits 0 and 1 control thestarting of the read and write processes respectively. A non-zero writeto the appropriate bit starts the process. Stop Bits 0 and 1 control thestopping of the read and write processes respectively. A non-zero writeto the appropriate bit stops the process. Stall This read-only statusbit comes from the LLFU's Stall flag. The Stall bit is valid when thewrite Go bit is set. A Stall value of 1 means that the LLFU is waitingfor the ADVANCE pulse from the MJI to continue. The processor can safelystart the LSGU for the first line once the Stall bit is set.ColorXfersRemaining[1-4] The number of 32-bit transfers remaining to beread into the specific Color[N] buffer.10.5 Controlling a Print

When controlling a print the processor 181 programs and starts the LLFU322 in read mode to ensure that the first line of the page istransferred to the buffer. When the interrupts arrive from the DMAcontroller 200, the processor 181 can switch LLFU buffers 330, 332, andprogram the MJI 320. The processor 181 then starts the LLFU 322 inread/write mode and starts the MJI 320. The processor 181 should thenwait a sufficient period of time to ensure that other connected printercontrollers have also started their LLFUs and MJIs (if there are noother connected printer controllers, the processor 181 must wait untilthe Stall bit of the LLFU 322 is set, a duration of 2 SegmentsPerXfercycles). The processor 181 can then program the LGSU 316, 318 to startthe synchronized print. As interrupts arrive from the DMA controllers200, the processor 181 can reprogram the DMA channels, swap LLFU buffers330, 332, and restart the LLFU 322 in read/write mode. Once the LLFU 332has effectively filled its pipeline, it will stall until the nextAdvance pulse from the MJI 320. The MJI 320 does not have to be touchedduring the print.

If for some reason the processor 181 wants to make any changes to theMJI 320 or LLFU 322 registers during an inter-line period it shouldensure that the current line has finished printing/loading by pollingthe status bits of the MJI 320 and the Go bits of the LLFU 322.

11 Generic Printer Driver

This section describes generic aspects of any host-based printer driverfor CePrint 10.

11.1 Graphics and Imaging Model

We assume that the printer driver is closely coupled with the hostgraphics system, so that the printer driver can provide device-specifichandling for different graphics and imaging operations, in particularcompositing operations and text operations.

We assume that the host provides support for color management, so thatdevice-independent color can be converted to CePrint-specific CMYK colorin a standard way, based on a user-selected CePrint-specific ICC(International Color Consortium) color profile. The color profile isnormally selected implicitly by the user when the user specifies theoutput medium in the printer (i.e. plain paper, coated paper,transparency, etc.). The page description sent to the printer 10 alwayscontains device-specific CMYK color.

We assume that the host graphics system renders images and graphics to anominal resolution specified by the printer driver, but that it allowsthe printer driver to take control of rendering text. In particular, thegraphics system provides sufficient information to the printer driver toallow it to render and position text at a higher resolution than thenominal device resolution.

We assume that the host graphics system requires random access to acontone page buffer at the nominal device resolution, into which itcomposites graphics and imaging objects, but that it allows the printerdriver to take control of the actual compositing—i.e. it expects theprinter driver to manage the page buffer.

11.2 Two-Layer Page Buffer

The printer's page description contains a 267 ppi contone layer and an800 dpi black layer. The black layer is conceptually above the contonelayer, i.e. the black layer is composited over the contone layer by theprinter. The printer driver therefore maintains a page buffer whichcorrespondingly contains a medium-resolution contone layer and ahigh-resolution black layer.

The graphics systems renders and composites objects into the page bufferbottom-up—i.e. later objects obscure earlier objects. This worksnaturally when there is only a single layer, but not when there are twolayers which will be composited later. It is therefore necessary todetect when an object being placed on the contone layer obscuressomething on the black layer.

When obscuration is detected, the obscured black pixels are compositedwith the contone layer and removed from the black layer. The obscuringobject is then laid down on the contone layer, possibly interacting withthe black pixels in some way. If the compositing mode of the obscuringobject is such that no interaction with the background is possible, thenthe black pixels can simply be discarded without being composited withthe contone layer. In practice, of course, there is little interactionbetween the contone layer and the black layer.

The printer driver specifies a nominal page resolution of 267 ppi to thegraphics system. Where possible the printer driver relies on thegraphics system to render image and graphics objects to the pixel levelat 267 ppi, with the exception of black text. The printer driver fieldsall text rendering requests, detects and renders black text at 800 dpi,but returns non-black text rendering requests to the graphics system forrendering at 267 ppi.

Ideally the graphics system and the printer driver manipulate color indevice-independent RGB, deferring conversion to device-specific CMYKuntil the page is complete and ready to be sent to the printer. Thisreduces page buffer requirements and makes compositing more rational.Compositing in CMYK color space is not ideal.

Ultimately the graphics system asks the printer driver to composite eachrendered object into the printer driver's page buffer. Each such objectuses 24-bit contone RGB, and has an explicit (or implicitly opaque)opacity channel.

The printer driver maintains the two-layer page buffer in three parts.The first part is the medium-resolution (267 ppi) contone layer. Thisconsists of a 24-bit RGB bitmap. The second part is a medium-resolutionblack layer. This consists of an 8-bit opacity bitmap. The third part isa high-resolution (800 dpi) black layer. This consists of a 1-bitopacity bitmap. The medium-resolution black layer is a subsampledversion of the high-resolution opacity layer. In practice, assuming thelow resolution is an integer factor n of the high resolution (e.g.n=800/267=3), each low-resolution opacity value is obtained by averagingthe corresponding n×n high-resolution opacity values. This correspondsto box-filtered subsampling. The subsampling of the black pixelseffectively antialiases edges in the high-resolution black layer,thereby reducing ringing artifacts when the contone layer issubsequently JPEG-compressed and decompressed.

The structure and size of the page buffer is illustrated in FIG. 47.

11.3 Compositing Model

For the purposes of discussing the page buffer compositing model, wedefine the following variables.

TABLE 41 Compositing variables variable description resolution format nmedium to high resolution scale — — factor C_(BgM) background contonelayer color medium 8-bit color component C_(ObM) contone object colormedium 8-bit color component α_(ObM) contone object opacity medium 8-bitopacity α_(FgM) medium-resolution foreground black medium 8-bit opacitylayer opacity α_(FgH) foreground black layer opacity high 1-bit opacityα_(TxH) black object opacity high 1-bit opacity

When a black object of opacity α_(T×H) is composited with the blacklayer, the black layer is updated as follows:α_(FgH) [x,y]←α _(FgH) [x,y]

α _(T×H) [x,y]  (Rule 1)

$\begin{matrix}\left. {\alpha_{FgM}\left\lbrack {x,y} \right\rbrack}\leftarrow{\frac{1}{n^{2}}{\sum\limits_{i = 0}^{n - 1}{\sum\limits_{j = 0}^{n - 1}{255{\alpha_{FgH}\left\lbrack {{{nx} + i},{{ny} + j}} \right\rbrack}}}}} \right. & \left( {{Rule}\mspace{14mu} 2} \right)\end{matrix}$

The object opacity is simply ored with the black layer opacity (Rule 1),and the corresponding part of the medium-resolution black layer isre-computed from the high-resolution black layer (Rule 2).

When a contone object of color C_(ObM) and opacity α_(ObM) is compositedwith the contone layer, the contone layer and the black layer areupdated as follows:C _(BgM) [x,y]←C _(BgM) [x,y](1−α_(FgM) [x,y]) if α_(ObM) [x,y]>0  (Rule3)α_(FgM) [x,y]←0 if α_(ObM) [x,y]>0  (Rule 4)α_(FgH) [x,y]←0 if α_(ObM) [x/n,y/n]>0  (Rule 5)C _(BgM) [x,y]←C _(BgM) [x,y](1−α_(ObM) [x,y])+C _(ObM) [x,y]α _(ObM)[x,y]  (Rule 6)

Wherever the contone object obscures the black layer, even if not fullyopaquely, the affected black layer pixels are pushed from the blacklayer to the contone layer, i.e. composited with the contone layer (Rule3) and removed from the black layer (Rule 4 and Rule 5). The contoneobject is then composited with the contone layer (Rule 6).

If a contone object pixel is fully opaque (i.e. α_(ObM)[x, y]=255), thenthere is no need to push the corresponding black pixels into thebackground contone layer (Rule 3), since the background contone pixelwill subsequently be completely obliterated by the foreground contonepixel (Rule 6).

11.4 Page Compression and Delivery

Once page rendering is complete, the printer driver converts the contonelayer to CePrint-specific CMYK with the help of color managementfunctions provided by the graphics system.

The printer driver then compresses and packages the black layer and thecontone layer into a CePrint page description as described in Section6.2. This page description is delivered to the printer 10 via thestandard spooler.

Note that the black layer is manipulated as a set of 1-bit opacityvalues, but is delivered to the printer 10 as a set of 1-bit blackvalues. Although these two interpretations are different, they share thesame representation, and so no data conversion is required.

The forward discrete cosine transform (DCT) is the costliest part ofJPEG compression. In current high-quality software implementations, theforward DCT of each 8×8 block requires 12 integer multiplications and 32integer additions. On typical modern general-purpose processors, aninteger multiplication requires 10 cycles, and an integer additionrequires 2 cycles. This equates to a total cost per block of 184 cycles.

The 26.4 MB contone layer consists of 432,538 JPEG blocks, giving anoverall forward DCT cost of about 80 Mcycles. At 150 MHz this equates toabout 0.5 seconds, which is 25% of the 2 second rendering time allowedper page.

A CE-oriented processor may have DSP support, in which case the presenceof single-cycle multiplication makes the JPEG compression timenegligible.

11.5 Banded Output

The printer control protocol supports the transmission of the page tothe printer 10 as a series of bands. If the graphics system alsosupports banded output, then this allows the printer driver to reduceits memory requirements by rendering the image one band at a time. Note,however, that rendering one band at a time can be more expensive thanrendering the whole page at once, since objects which span multiplebands have to be handled multiple times.

Although banded rendering can be used to reduce memory requirements inthe printer driver, buffers for two bands are still required. One bufferis required for the band being transmitted to the printer 10; anotherbuffer is required for the band being rendered. A single buffer maysuffice if the connection between the host processor and printer issufficiently fast. The band being transmitted to the printer may also bestored on disk, if a disk drive is present in the system, and onlyloaded into memory block-by-block during transmission.

12 Windows 9x/NT/CE Printer Driver

12.1 Windows 9x/NT/CE Printing System

In the Windows 9x/NT/CE printing system, a printer is a graphics device,and an application communicates with it via the graphics deviceinterface (GDI). The printer driver graphics DLL (dynamic link library)implements the device-dependent aspects of the various graphicsfunctions provided by GDI.

The spooler handles the delivery of pages to the printer, and may resideon a different machine to the application requesting printing. Itdelivers pages to the printer via a port monitor which handles thephysical connection to the printer. The optional language monitor is thepart of the printer driver which imposes additional protocol oncommunication with the printer, and in particular decodes statusresponses from the printer on behalf of the spooler.

The printer driver user interface DLL implements the user interface forediting printer-specific properties and reporting printer-specificevents.

The structure of the Windows 9x/NT/CE printing system is illustrated inFIG. 48.

The printer driver language monitor and user interface DLL mustimplement the implement the relevant aspects of the printer controlprotocol described in Section 6. The remainder of this section describesthe design of the printer driver graphics DLL. It should be read inconjunction with the appropriate Windows 9x/NT/CE DDK documentation.

12.2 Windows 9×/NT/CE Graphics Device Interface (GDI)

GDI provides functions which allow an application to draw on a devicesurface, i.e. typically an abstraction of a display screen or a printedpage. For a raster device, the device surface is conceptually a colorbitmap. The application can draw on the surface in a device-independentway, i.e. independently of the resolution and color characteristics ofthe device.

The application has random access to the entire device surface. Thismeans that if a memory-limited printer device requires banded output,then GDI must buffer the entire page's GDI commands and replay themwindowed into each band in turn. Although this provides the applicationwith great flexibility, it can adversely affect performance. GDIsupports color management, whereby device-independent colors provided bythe application are transparently translated into device-dependentcolors according to a standard ICC (International Color Consortium)color profile of the device. A printer driver can activate a differentcolor profile depending, for example, on the user's selection of papertype on the driver-managed printer property sheet.

GDI supports line and spline outline graphics (paths), images, and text.Outline graphics, including outline font glyphs, can be stroked andfilled with bit-mapped brush patterns. Graphics and images can begeometrically transformed and composited with the contents of the devicesurface. While Windows 95/NT4 provides only boolean compositingoperators, Windows 98/NT5 provides proper alpha-blending.

12.3 Printer Driver Graphics DLL

A raster printer can, in theory, utilize standard printer drivercomponents under Windows 9x/NT/CE, and this can make the job ofdeveloping a printer driver trivial. This relies on being able to modelthe device surface as a single bitmap. The problem with this is thattext and images must be rendered at the same resolution. This eithercompromises text resolution, or generates too much output data,compromising performance. As described earlier, CePrint's approach is torender black text and images at different resolutions, to optimize thereproduction of each. The printer driver is therefore implementedaccording to the generic design described in Section 11.

The driver therefore maintains a two-layer three-part page buffer asdescribed in Section 11.2, and this means that the printer driver musttake over managing the device surface, which in turn means that it mustmediate all GDI access to the device surface.

12.3.1 Managing the Device Surface

A graphics driver must support a number of standard functions, includingthe following:

TABLE 42 Standard graphics driver interface functions functiondescription DrvEnableDriver Initial entry point into the driver graphicsDLL. Returns addresses of functions supported by the driver.DrvEnablePDEV Creates a logical representation of a physical device withwhich the driver can associate a drawing surface. DrvEnableSurfaceCreates a surface to be drawn on, associated with a given PDEV.

DrvEnablePDEV indicates to GDI, via the flGraphicsCaps member of thereturned DEVINFO structure, the graphics rendering capabilities of thedriver. This is discussed further below.

DrvEnableSurface creates a device surface consisting of two conceptuallayers and three parts: the 267 ppi contone layer 24-bit RGB color, the267 ppi black layer 8-bit opacity, and the 800 dpi black layer 11-bitopacity. The virtual device surface which encapsulates these two layershas a nominal resolution of 267 ppi, so this is the resolution at whichGDI operations take place.

Although the aggregate page buffer requires about 34 MB of memory, thesize of the page buffer can be reduced arbitrarily by rendering the pageone band at a time, as described in Section 12.3.4.

A printer-specific graphics driver must also support the followingfunctions:

TABLE 43 Required printer driver functions function descriptionDrvStartDoc Performs any start-of-document handling DrvStartPage Handlesthe start of a new page. DrvSendPage Sends the current page to theprinter via the spooler. DrvEndDoc Performs any end-of-documenthandling.

DrvStartDoc sends the start document command to the printer, andDrvEndDoc sends the end document command.

DrvStartPage sends the start page command with the page header to theprinter.

DrvSendPage converts the contone layer from RGB to CMYK usingGDI-provided color management functions, compresses both the contone andblack layers, and sends the compressed page as a single band to theprinter (in a page band command).

Both DrvStartPage and DrvSendPage use EngWritePrinter to send data tothe printer via the spooler.

Managing the device surface and mediating GDI access to it means thatthe printer driver must support the following additional functions:

TABLE 44 Required graphics driver functions for a device-managed surfacefunction description DrvCopyBits Translates between device-managedraster surfaces and GDI-managed standard-format bitmaps. DrvStrokePathStrokes a path. DryPaint Paints a specified region. DrvTextOut Renders aset of glyphs at specified positions.

Copying images, stroking paths and filling regions all occur on thecontone layer, while rendering solid black text occurs on the bi-levelblack layer. Furthermore, rendering non-black text also occurs on thecontone layer, since it isn't supported on the black layer. Conversely,stroking or filling with solid black can occur on the black layer (if weso choose).

Although the printer driver is obliged to hook the aforementionedfunctions, it can punt function calls which apply to the contone layerback to the corresponding GDI implementations of the functions, sincethe contone layer is a standard-format bitmap. For every DrvXxx functionthere is a corresponding EngXxx function provided by GDI.

As described in Section 11.2, when an object destined for the contonelayer obscures pixels on the black layer, the obscured black pixels mustbe transferred from the black layer to the contone layer before thecontone object is composited with the contone layer. The key to thisprocess working is that obscuration is detected and handled in thehooked call, before it is punted back to GDI. This involves determiningthe pixel-by-pixel opacity of the contone object from its geometry, andusing this opacity to selectively transfer black pixels from the blacklayer to the contone layer as described in Section 11.2.

12.3.2 Determining Contone Object Geometry

It is possible to determine the geometry of each contone object beforeit is rendered and thus determine efficiently which black pixels itobscures. In the case of DrvCopyBits and DrvPaint, the geometry isdetermined by a clip object (CLIPOBJ), which can be enumerated as a setof rectangles.

In the case of DrvStrokePath, things are more complicated. DrvStrokePathsupports both straight-line and Bézier-spline curve segments, andsingle-pixel-wide lines and geometric-wide lines. The first step is toavoid the complexity of Bézier-spline curve segments and geometric-widelines altogether by clearing the corresponding capability flags(GCAPS_BEZIERS and GCAPS_GEOMETRICWIDE) in the flGraphicsCaps member ofthe driver's DEVINFO structure. This causes GDI to reformulate suchcalls as sets of simpler calls to DrvPaint. In general, GDI gives adriver the opportunity to accelerate high-level capabilities, butsimulates any capabilities not provided by the driver.

What remains is simply to determine the geometry of a single-pixel-widestraight line. Such a line can be solid or cosmetic. In the latter case,the line style is determined by a styling array in the specified lineattributes (LINEATTRS). The styling array specifies how the linealternates between being opaque and transparent along its length, and sosupports various dashed line effects etc.

When the brush is solid black, straight lines can also usefully berendered to the black layer, though with the increased width implied bythe 800 dpi resolution.

12.3.3 Rendering Text

In the case of a DrvTextOut, things are also more complicated. Firstly,the opaque background, if any, is handled like any other fill on thecontone layer (see DrvPaint). If the foreground brush is not black, orthe mix mode is not effectively opaque, or the font is not scalable, orthe font indicates outline stroking, then the call is punted toEngTextOut, to be applied to the contone layer. Before the call ispunted, however, the driver determines the geometry of each glyph byobtaining its bitmap (via FONTOBJ_cGetGlyphs), and makes the usualobscuration check against the black layer.

If punting a DrvTextOut call is not allowed (the documentation isambiguous), then the driver should disallow complex text operations.This includes disallowing outline stroking (by clearing theGCAPS_VECTOR_FONT capability flag), and disallowing complex mix modes(by clearing the GCAPS_ARBMIXTXT capability flag).

If the foreground brush is black and opaque, and the font is scalableand not stroked, then the glyphs are rendered on the black layer. Inthis case the driver determines the geometry of each glyph by obtainingits outline (again via FONTOBJ_cGetGlyphs, but as a PATHOBJ). The driverthen renders each glyph from its outline at 800 dpi and writes it to theblack layer. Although the outline geometry uses device coordinates (i.e.at 267 ppi), the coordinates are in fixed point format with plenty offractional precision for higher-resolution rendering.

Note that strikethrough and underline rectangles are added to the glyphgeometry, if specified. The driver must set the GCAPS_HIGHRESTEXT flagin the DEVINFO to request that glyph positions (again in 267 ppi devicecoordinates) be supplied by GDI in high-precision fixed-point format, toallow accurate positioning at 800 dpi. The driver must also provide animplementation of the DrvGetGlyphMode function, so that it can indicateto GDI that glyphs should be cached as outlines rather than bitmaps.Ideally the driver should cache rendered glyph bitmaps for efficiency,memory allowing. Only glyphs below a certain point size should becached.

12.3.4 Banded Output

As described in Section 6, the printer control protocol supports bandedoutput by breaking the page description into a page header and a numberof page bands. GDI supports banded output to a printer to cater forprinter drivers and printers which have limited internal buffer memory.

GDI can handle banded output without application involvement. GDI simplyrecords all the graphics operations performed by the application in ametafile, and then replays the entire metafile to the printer driver foreach band in the page. The printer driver must clip the graphicsoperations to the current band, as usual. Banded output can be moreefficient if the application takes note of the RC_BANDING bit in thedriver's raster capabilities (returned by GetDeviceCaps when called withthe RASTERCAPS index) and only performs graphics operations relevant toeach band.

If banded output is desired because memory is limited, then the printerdriver must enable banding by calling EngMarkBandingSurface inDrvEnableSurface. It must also support the following additionalfunctions:

TABLE 45 Required printer driver functions function descriptionDrvStartBanding Prepares the driver for banding and returns the originof the first band. DrvNextBand Sends the current band to the printer andreturns the origin of the next band, if any.

Like DrvSendPage, DrvNextBand converts the contone layer from RGB toCMYK using GDI-provided color management functions, compresses both thecontone and black layers, and sends the compressed page band to theprinter (in a page band command).

It uses EngWritePrinter to send the band data to the printer via thespooler.

1. A controller for controlling printing on both surfaces of a sheet ofprint media, the controller including: a first print controller forcontrolling printing by a page width printhead of a first print engine;a second print controller for controlling printing by a page widthprinthead of a second print engine substantially simultaneously with theprinting by the printhead of the first print engine; a firstcommunications link interconnecting the first print controller and thesecond print controller for synchronising the first and second printcontrollers; and a second communications link interconnecting the firstprint controller with a host system via which the first print controllerreceives from the host system descriptions of pages to be printed onsaid surfaces of the sheet of print media by the first and second printengines.
 2. The controller as claimed in claim 1 wherein the first printcontroller is a master print controller and the second print controlleris a slave print controller operable under command of the master printcontroller on receipt of signals via the first communications link. 3.The controller as claimed in claim 2 in which the first communicationslink is a bi-directional link enabling the transmission of data from theslave print controller to the master print controller.
 4. The controlleras claimed in claim 2 in which print synchronisation is achieved by themaster print controller controlling a printing operation of the slaveprint controller.
 5. A controller as claimed in claim 1, wherein thefirst print controller is arranged to transmit the description of pagesto be printed by the second print engine to the second print controllervia the first communications link.
 6. A controller as claimed in claim2, wherein the master print controller has the second communicationslink connected to it to present a unified view to the host system tomask the presence of the slave print controller.
 7. A controller asclaimed in claim 2, wherein the master print controller prints describedpages on a rear surface of the print media and the slave printcontroller prints on a front surface of the print media so that themaster print controller always has a page buffer available for a pagedescription destined for the slave print controller.
 8. A controller asclaimed in claim 4, wherein printhead interfaces of the master and slaveprint controllers are synchronised to a shared line synchronisationsignal generated by the master print controller.
 9. A method ofcontrolling printing on both surfaces of a sheet of print media, themethod including the steps of: receiving data relating to a first pageto be printed in a first print controller of a first print engine;transmitting the data relating to the first page from the first printcontroller to a second print controller of a second print engine;receiving data relating to a second page to be printed in the firstprint controller; and controlling printing by the print engines undercommand of the first print controller to achieve synchronisation ofprinting of the pages by the first print controller and the second printcontroller on rear and front surfaces of the print media, respectively.10. The method as claimed in claim 9 which includes synchronisingprinthead interfaces of both print controllers by means of a shared linesynchronisation signal generated by the first print controller which isa master print controller.
 11. The method as claimed in claim 10 whichincludes transmitting data relating to the pages to be printed from ahost system to the master print controller by means of a hostcommunications link, the master print controller determining whether ornot said data are to be routed to the second print controller which is aslave print controller.
 12. The method as claimed in claim 11 whichincludes receiving said data in its entirety in a memory of the masterprint controller before forwarding it to the slave print controller. 13.The method as claimed in claim 11 which includes selecting the masterprint controller to print the rear surface of the print media to ensurethat the master print controller always has a page buffer available fora page description destined for the slave print controller.
 14. Themethod as claimed in claim 11 which includes, periodically, transmittingpredetermined data from the slave print controller to the master printcontroller.
 15. A controller for controlling printing on both surfacesof a sheet of print media, the controller including: a first printcontroller for controlling printing by a page width printhead of a firstprint engine; a second print controller for controlling printing by apage width printhead of a second print engine substantiallysimultaneously with the printing by the printhead of the first printengine; a first communications link interconnecting the first printcontroller and the second print controller for synchronising the printcontrollers; and a second communications link interconnecting at leastone of the print controllers with a host system for receipt from thehost system of descriptions of pages to be printed on said surfaces ofthe sheet of print media by the print engines; wherein the first printcontroller is a master print controller and the second print controlleris a slave print controller operable under command of the master printcontroller on receipt of signals via the first communications link, andthe second communications link is connected to the master printcontroller to present a unified view to the host system to mask thepresence of the slave print controller.
 16. A controller for controllingprinting on both surfaces of a sheet of print media, the controllerincluding: a first print controller for controlling printing by a pagewidth printhead of a first print engine; a second print controller forcontrolling printing by a page width printhead of a second print enginesubstantially simultaneously with the printing by the printhead of thefirst print engine; a first communications link interconnecting thefirst print controller and the second print controller for synchronisingthe print controllers; and a second communications link interconnectingat least one of the print controllers with a host system for receiptfrom the host system of descriptions of pages to be printed on saidsurfaces of the sheet of print media by the print engines; wherein thefirst print controller is a master print controller and the second printcontroller is a slave print controller operable under command of themaster print controller on receipt of signals via the firstcommunications link, and the master print controller prints describedpages on a rear surface of the print media with the slave printcontroller printing on a front surface of the print media so that themaster print controller always has a page buffer available for a pagedescription destined for the slave print controller.
 17. A controllerfor controlling printing on both surfaces of a sheet of print media, thecontroller including: a first print controller for controlling printingby a page width printhead of a first print engine; a second printcontroller for controlling printing by a page width printhead of asecond print engine substantially simultaneously with the printing bythe printhead of the first print engine; a first communications linkinterconnecting the first print controller and the second printcontroller for synchronising the first and second print controllers; anda second communications link interconnecting the first print controllerwith a host system via which the first print controller receives fromthe host system descriptions of pages to be printed on said surfaces ofthe sheet of print media by the first and second print engines; whereinprinthead interfaces of the first and second print controllers aresynchronised to a shared line synchronisation signal generated by thefirst print controller.
 18. A controller for controlling printing onboth surfaces of a sheet of print media, the controller including: afirst print controller for controlling printing by a page widthprinthead of a first print engine; a second print controller forcontrolling printing by a page width printhead of a second print enginesubstantially simultaneously with the printing by the printhead of thefirst print engine; a first communications link interconnecting thefirst print controller and the second print controller for synchronisingthe print controllers; and a second communications link interconnectingat least one of the print controllers with a host system for receiptfrom the host system of descriptions of pages to be printed on saidsurfaces of the sheet of print media by the print engines; wherein thefirst print controller is a master print controller and the second printcontroller is a slave print controller operable under command of themaster print controller on receipt of signals via the firstcommunications link, the printhead interfaces of both print controllersare synchronised to a shared line synchronisation signal generated byone of the print controllers, and wherein print synchronisation isachieved by the master print controller controlling a printing operationof the slave print controller.